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).
Doesn't look like anything is using this.  Can we get rid of it, or did you have other plans for it?