Handle commandline parsing errors gracefully

Handle commandline parsing errors gracefully

The commandline option parser raises SystemExit after having explained
to the user what she did wrong. A recent refactoring introduced a
catch-all "except:" at the top level, which caught the SystemExit and
printed a traceback.

Let's just simply exit instead, like we used to.

Signed-off-by: Karl Hasselström <kha@treskal.com>