librbd: report better errors when failing to enable mirroring on an image

[pkalever]

Attempting to enable mirroring on 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>

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[idryomov]

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2024444

No need to mention this, the upstream tracker link is enough. BZs are often either completely inaccessible or most of the comments are private.

[idryomov]

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