Description of problem:
When attempting to background remove an RBD image when it's still in-use (i.e. it has a child image attached), the remove will correctly fail but (1) it leaves an apparently stuck progress notification and (2) the MGR logs shows the (expected) failure every thirty seconds while it retries.

Note that is purely a progress status and log issue. Once the image can be deleted (since the associated child images are flattened or removed), the removal will correctly proceed.

Version-Release number of selected component (if applicable):
RHCS 4.1

How reproducible:
100%

Steps to Reproduce:
1. Create a parent and child image
2. Run "rbd task add [trash] remove <image-[id-]spec>"
3. 

Actual results:

Progress:

sh-4.4# ceph -s

< .... SNIP ... >
  progress:
    Removing image ocs-storagecluster-cephblockpool/a10a894b726cc from trash
      [..............................]
    Removing image ocs-storagecluster-cephblockpool/a10a87ad46bc5 from trash
      [..............................]
    Removing image ocs-storagecluster-cephblockpool/a10a862e83ce1 from trash
      [..............................]

Logs:

debug 2020-11-17 13:02:54.211 7f44e544e700 -1 librbd::SnapshotRemoveRequest: 0x558a65235a20 should_complete: encountered error: (16) Device or resource busy
debug 2020-11-17 13:02:54.211 7f44e544e700 -1 librbd::image::PreRemoveRequest: 0x558a64e78f20 handle_remove_snapshot: failed to auto-prune snapshot 16: (16) Device or resource busy
debug 2020-11-17 13:02:54.215 7f44e32ca700  0 mgr[rbd_support] execute_task: [errno 39] error deleting image from trash

Expected results:
Images that cannot be removed due to dependencies should not be listed in the progress and we should avoid logging errors 

Additional info: