If sys.exit is called or SystemExit raised, don't display success

If sys.exit is called, which raises SystemExit, the finally at the
end of the installer was treating it as a successful install and
displaying messages to the user. Catch this exception and mark
the install as failed to prevent this.

https://fedorahosted.org/ipsilon/ticket/66

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>