mgr/rbd_support: bail out when snapshot mirroring is not enabled

mgr/rbd_support: bail out when snapshot mirroring is not enabled

before this change, we continue on and try to get mirror info of
specified image after calling close_image(), even the snapshot mirroring
is not enabled.

after this change, we bail out after calling close_image(). this
behavior is consistent with other places where we handle errors.

Fixes: https://tracker.ceph.com/issues/49355
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 8372d9370b530f953acd5bd6502c46fd11f43109)