A bad system repo can cause lorax-composer to fail to start. Instead of
a traceback log the error and exit.
(note that the exit still results in an OSError traceback due to part of
it running as root, this needs to be addressed in another commit).
(cherry picked from commit 49380b4b49)
This no longer uses the enabled configuration setting to select repos to
use. It uses everything in the repo_dir, and if system repos have not
been disabled it copies them into the repo_dir at startup, overwriting
the previous copy.