Raising as requested by abayer on IRC.

I experienced an issue with a shared library where @Grab failed with the following in the console log:

hudson.remoting.ProxyException: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
General error during conversion: Error grabbing Grapes -- [unresolved dependency: com.rabbitmq#amqp-client;4.1.0: not found]

This is likely due to the Jenkins instance being behind a proxy, as running the same shared library on another instance without need of a proxy was able to proceed beyond this point.

I'm not sure what Andrew had in mind for improving these diagnostics. Perhaps instead of "not found" something like "unable to establish connect to..." would be more likely to indicate a proxy or other networking issue.