Exit status should not be 0 on error

[aburgh]

This project looks interesting, I look forward to using it in some projects. Thanks for your work.

When exiting due to a parsing error, argparse exits with status 0, but it should probably return EXIT_FAILURE (from stdlib.h) or EX_USAGE (from sysexits.h). If the user passes "-h", then 0 is correct, in my opinion.

[cofyc]

Thanks for you suggestion!