mgr/volumes: Add clone failure reason in clone status

mgr/volumes: Add clone failure reason in clone status

Add the clone failure reason in the clone status.
The sample output is as below:

$ ceph fs clone status cephfs clone_0
{
  "status": {
    "state": "failed",
    "source": {
      "volume": "cephfs",
      "subvolume": "subvolume_0",
      "snapshot": "snapshot_0",
      "size": "52428800"
    },
    "failure": {
      "errno": "2",
      "error_msg": "snapshot 'snapshot_0' does not exist"
    }
  }
}

Fixes: https://tracker.ceph.com/issues/55190
Signed-off-by: Kotresh HR <khiremat@redhat.com>