mgr/volumes: unregister job upon async threads exception

mgr/volumes: unregister job upon async threads exception

If the async threads hit a temporary exception the job is
never unregistered and therefore gets skipped by the async
threads on subsequent scans.

Patrick hit this in nautilus when one of the purge threads
hit an exception when trying to log a message. The trash
entry was never picked up again by the purge threads.

Fixes: http://tracker.ceph.com/issues/44293
Signed-off-by: Venky Shankar <vshankar@redhat.com>