If I enter the schedule in an wrong format, I get an exception instead of an
error message:

javax.servlet.ServletException: antlr.ANTLRException: Invalid input: "20 1 * * *
1 2": line 1:11: expecting EOF, found ' '
org.kohsuke.stapler.Stapler.invoke(Stapler.java:269)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:240)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:260)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:198)
org.kohsuke.stapler.Stapler.service(Stapler.java:72)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

javax.servlet.ServletException: antlr.ANTLRException: Invalid input: "20 1 * * *
1 2": line 1:11: expecting EOF, found ' '
hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:500)
hudson.model.Project.doConfigSubmit(Project.java:173)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:585)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:50)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:63)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:260)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:240)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:260)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:198)
org.kohsuke.stapler.Stapler.service(Stapler.java:72)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)