options.py: o.fatal(): print error after, not before, usage message.

options.py: o.fatal(): print error after, not before, usage message.

git prints the error *before* the usage message, but the more I play with
it, the more I'm annoyed by that behaviour.  The usage message can be pretty
long, and the error gots lost way above the usage message.  The most
important thing *is* the error, so let's print it last.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>