Using sys.exit in a management command makes it impossible
to unit test the code in question. The correct approach to do the same
thing in Django management commands is to raise CommandError.
Followup of b570c0dafa
When soft deactivation is run for in "auto" mode (no emails are
specified and all users inactive for specified number of days are
deactivated), catch-up is also run in the "auto" mode if
AUTO_CATCH_UP_SOFT_DEACTIVATED_USERS is True.
Automatically catching up soft-deactivated users periodically would
ensure a good user experience for returning users, but on some servers
we may want to turn off this option to save on some disk space.
Fixes#8858, at least for the default configuration, by eliminating
the situation where there are a very large number of messages to recover.
