rbd: improve log msgs when failing to enable mirroring on image

rbd: improve log msgs when failing to enable mirroring on image

Attempting to enable mirroring on an an image within a namespace bails
out without any useful msg.

$ rbd mirror image enable mi_pool/ns_0/big snapshot
2021-11-17T08:33:21.321+0000 7f4b36f9c2c0 -1 librbd::api::Mirror:
  image_enable: cannot enable mirroring in the current pool mirroring mode

Fixes: https://tracker.ceph.com/issues/58895
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
(cherry picked from commit 828a0dea2d40ba0c1aa0a9d4b211f0fabc854350)