SVN plugin can work through proxy, but it doesn't bother whether proxy works or not.
Once our proxy wasn't working correctly and the job freezed, doing nothing and leaving console log empty.
This time we had some problems with connection (i.e. proxy worked but there were no access to the internet for a limited time) and job failed with such an error:

ERROR: Failed to update svn://svn-url-here
org.tmatesoft.svn.core.SVNException: svn: E204900: C:\path-to-project-here\
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl$1.handleStatus(UpdateWithCleanUpdater.java:78)
at org.tmatesoft.svn.core.wc.SVNStatusClient$1.receive(SVNStatusClient.java:356)
at org.tmatesoft.svn.core.wc.SVNStatusClient$1.receive(SVNStatusClient.java:353)
at org.tmatesoft.svn.core.wc2.SvnReceivingOperation.receive(SvnReceivingOperation.java:78)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.handleStatus(SvnOldGetStatus.java:37)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.sendUnversionedStatus(SVNStatusEditor.java:361)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.getDirStatus(SVNStatusEditor.java:209)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.handleDirEntry(SVNStatusEditor.java:321)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.getDirStatus(SVNStatusEditor.java:256)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.closeEdit(SVNStatusEditor.java:114)
at org.tmatesoft.svn.core.internal.wc16.SVNStatusClient16.doStatus(SVNStatusClient16.java:430)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.run(SvnOldGetStatus.java:22)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.run(SvnOldGetStatus.java:13)
at org.tmatesoft.svn.core.internal.wc2.SvnOperationRunner.run(SvnOperationRunner.java:20)
at org.tmatesoft.svn.core.wc2.SvnOperationFactory.run(SvnOperationFactory.java:1238)
at org.tmatesoft.svn.core.wc2.SvnOperation.run(SvnOperation.java:294)
at org.tmatesoft.svn.core.wc.SVNStatusClient.doStatus(SVNStatusClient.java:360)
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl.preUpdate(UpdateWithCleanUpdater.java:66)
at hudson.scm.subversion.UpdateUpdater$TaskImpl.perform(UpdateUpdater.java:151)
at hudson.scm.subversion.WorkspaceUpdater$UpdateTask.delegateTo(WorkspaceUpdater.java:161)
at hudson.scm.SubversionSCM$CheckOutTask.perform(SubversionSCM.java:1003)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:984)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:960)
at hudson.FilePath.act(FilePath.java:922)
at hudson.FilePath.act(FilePath.java:895)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:909)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:844)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1254)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:624)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:530)
at hudson.model.Run.execute(Run.java:1732)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:529)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:234)
Caused by: svn: E204900: C:\path-to-project-here\
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:109)
... 35 more
Caused by: java.nio.file.DirectoryNotEmptyException: C:\path-to-project-here\
at sun.nio.fs.WindowsFileSystemProvider.implDelete(Unknown Source)
at sun.nio.fs.AbstractFileSystemProvider.delete(Unknown Source)
at java.nio.file.Files.delete(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at hudson.Util.deleteFile(Util.java:247)
at hudson.Util.deleteRecursive(Util.java:310)
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl$1.handleStatus(UpdateWithCleanUpdater.java:74)
... 34 more
ERROR: Subversion update failed
java.io.IOException
at hudson.scm.subversion.UpdateUpdater$TaskImpl.perform(UpdateUpdater.java:211)
at hudson.scm.subversion.WorkspaceUpdater$UpdateTask.delegateTo(WorkspaceUpdater.java:161)
at hudson.scm.SubversionSCM$CheckOutTask.perform(SubversionSCM.java:1003)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:984)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:960)
at hudson.FilePath.act(FilePath.java:922)
at hudson.FilePath.act(FilePath.java:895)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:909)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:844)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1254)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:624)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:530)
at hudson.model.Run.execute(Run.java:1732)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:529)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:234)
Caused by: hudson.scm.subversion.UpdaterException: failed to perform svn update
... 17 more
Caused by: org.tmatesoft.svn.core.SVNException: svn: E204900: C:\path-to-project-here\
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl$1.handleStatus(UpdateWithCleanUpdater.java:78)
at org.tmatesoft.svn.core.wc.SVNStatusClient$1.receive(SVNStatusClient.java:356)
at org.tmatesoft.svn.core.wc.SVNStatusClient$1.receive(SVNStatusClient.java:353)
at org.tmatesoft.svn.core.wc2.SvnReceivingOperation.receive(SvnReceivingOperation.java:78)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.handleStatus(SvnOldGetStatus.java:37)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.sendUnversionedStatus(SVNStatusEditor.java:361)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.getDirStatus(SVNStatusEditor.java:209)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.handleDirEntry(SVNStatusEditor.java:321)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.getDirStatus(SVNStatusEditor.java:256)
at org.tmatesoft.svn.core.internal.wc.SVNStatusEditor.closeEdit(SVNStatusEditor.java:114)
at org.tmatesoft.svn.core.internal.wc16.SVNStatusClient16.doStatus(SVNStatusClient16.java:430)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.run(SvnOldGetStatus.java:22)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldGetStatus.run(SvnOldGetStatus.java:13)
at org.tmatesoft.svn.core.internal.wc2.SvnOperationRunner.run(SvnOperationRunner.java:20)
at org.tmatesoft.svn.core.wc2.SvnOperationFactory.run(SvnOperationFactory.java:1238)
at org.tmatesoft.svn.core.wc2.SvnOperation.run(SvnOperation.java:294)
at org.tmatesoft.svn.core.wc.SVNStatusClient.doStatus(SVNStatusClient.java:360)
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl.preUpdate(UpdateWithCleanUpdater.java:66)
at hudson.scm.subversion.UpdateUpdater$TaskImpl.perform(UpdateUpdater.java:151)
... 16 more
Caused by: svn: E204900: C:\path-to-project-here\
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:109)
... 35 more
Caused by: java.nio.file.DirectoryNotEmptyException: C:\path-to-project-here\
at sun.nio.fs.WindowsFileSystemProvider.implDelete(Unknown Source)
at sun.nio.fs.AbstractFileSystemProvider.delete(Unknown Source)
at java.nio.file.Files.delete(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at hudson.Util.deleteFile(Util.java:247)
at hudson.Util.deleteRecursive(Util.java:310)
at hudson.scm.subversion.UpdateWithCleanUpdater$TaskImpl$1.handleStatus(UpdateWithCleanUpdater.java:74)
... 34 more

So from my point of view it would be a nice improvement to (somehow) check availability before doing svn up or something else.