Use exit code 1 on failure when pulling

[fmauch]

Currently, the pull command returns 0 even when a failure occurred.

My usecase is having this running in a systemd timer job and every now and then it happens, that I shutdown my machine while there is a pull command running. This results in a lockfile being present which prevent future runs from succeeding.

However, since that case is caught in an exception there is the error output, but the job itself returns success (exit code 0).

This PR exits with code 1 as done in _try_load_config, as this was the simplest to implement. Maybe a nicer approach would be to raise a SystemExit instead but that will not directly put it to the critical log. Please let me know what you would prefer.

[ryneeverett]

Looking through the code base, we seem to be evenly split between log.critical/sys.exit and SystemExit. I'm not aware if there's a substantive difference so I don't have a preference.

[fmauch]

Perfect, then let's just leave it like that. Thanks for the quick review!