get script name from module string; raise command exception

1. we lose the final script name without modifying sys.argv if using a wrapper to run commands.
For example:
./manage.py maven rebuild_index --traceback

In this case "maven" is a script name as well as "rebuild_index", but MMC will store two log entries for the "maven" script instead of one log for each script.

https://github.com/saippuakauppias/django-maven

2. If command exception is not re-raised we lose it in our logs (sentry, for example)