diffconfig: Gracefully exit if the default config files are not present

Handle gracefully the instance where config files are not present.
Compatible with python versions 2.5, 2.6 and 2.7.
The try/except is forward compatible with python version 3 once the entire script is ported.

Signed-off-by: Mike Pagano <mpagano@gentoo.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>