Fix help with debug

This should fix issue #181.

There is a small behaviour change, before this submit:
- an exception was thrown (and caught by the ruby runtime), resulting in a non-zero return code when executing _somecommand --help_ with GLI_DEBUG set to true;
- after this submit, no exception is thrown in that case.

Exit code (when not using GLI_DEBUG) stays the same, output stays the same (minus the exception stack trace).
Oops, scratch that, this is fine.