Return non-zero exit code on exception

[mjcheetham]

Return a non-zero program exit code when an unhandled exception is encountered at the top-level command line handling.

Fixes #440

[dscho]

Makes sense. Are there potentially other code sites that would need this kind of treatment, too?

[mjcheetham]

Are there potentially other code sites that would need this kind of treatment, too?

Almost all code is contained within System.CommandLine callbacks, so either the commands return the exit code (already do) or the exception handler sets the exit code (this PR).