Let's check return value of mirror_start_job to check for failure
instead of local_err.

Rename ret to job, as ret is usually integer variable.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20210202124956.63146-7-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>