mgr/volumes: fix `subvolume group rm` error message

mgr/volumes: fix `subvolume group rm` error message

Currently, if we try to delete subvolumegroup using `fs subvolumegroup rm`
when there's one or more subvolume(s) present under that subvolumegroup we
see the error something like :
`Error ENOTEMPTY: error in rmdir /volumes/group1`
which causes confusion. Make it more descriptive

Fixes: https://tracker.ceph.com/issues/62968
Signed-off-by: Neeraj Pratap Singh <neesingh@redhat.com>
(cherry picked from commit fca554cee093771646a0c4a37827d7fe3fe95a4a)