mojo should not return 0 when printing usage information

[Neale Pickett]

We just finished debugging an issue wherein mojo was returning 0 when we passed the wrong arguments, giving our CI run no indication of a problem:

ubuntu@juju-prod45-ues-ci-jenkaas-is-mojo-ci-machine-2:/var/lib/jenkins$ /usr/bin/mojo run --project mojo-ue-jenkaas --series trusty --stage ue/mojo-ue-jenkaas/staging 351-ue-mojo-ue-jenkaas-staging &>/dev/null; echo $?
0

Mojo should return non-zero when command-line arguments are incorrect. EX_USAGE (64) as defined in /usr/include/sysexits.h seems appropriate.

[Tom Haddon (mthaddon)]

This appears to be argparser behaviour. See http://stackoverflow.com/questions/5943249/python-argparse-and-controlling-overriding-the-exit-status-code and https://www.python.org/dev/peps/pep-0389/#discussion-sys-stderr-and-sys-exit particularly.