Solution implemented with MGNLUI-3062 "does the job" in preventing and notifying when trying to delete folders with groups/roles that are still in use, but it hasn't quite met the UI requirements, in particular:

• This is not an alert, but a notification. Alerts have an [ OK ] button.
• This is not of level WARNING (yellow) but ERROR (red).
• When folder is not empty, I should get the alert as soon as I click 'Delete folder' in the action bar, not after the confirmation.
• Wording is not as specified in the comment. Please also use separate keys for roles VS. groups.

Original specification comment:

For single groups (resp. roles), if I attempt to delete a group which is still in use, I get a warning alert:
"This group still contains users. Please empty the group first. [ OK ]"

Likewise for group folders (resp. role folders). I get a warning alert:
"This folder contains groups which are still in use. Please make sure no users are assigned to these first. [ OK ]"

If the folder contains only unused items, then we get the confirmation dialog as usual.

Meanwhile, current implementation has the advantage of showing details as to which assignment is currently preventing deletion.
We should think about how we want to display these details, both in terms of what can be done right now, and what should be done in the future.