This is a follow up to #3040

Since we're normally running through systemd and, after #3040, systemd will restart us on errors, we should be more systematic about our exit codes.

At present, the following situations will lead to kojid exiting with a zero status.

• SIGTERM / KeyboardInterrupt / SystemExit
• AuthExpired exception
• shutdown task handler

I think AuthExpired case should be an error (and hence a restart condition at the systemd level). The others can probably remain as-is.