Update permission downgrade message (rebased onto develop)



This is the same as gh-2501 but rebased onto develop.

----

Updated the warning message shown when a group is going to be downgraded to Read-Only; to match OMERO.web, see PR #2485
Showing an error dialog if downgrade really fails, has to be tackled with another PR for 5.0.3 as it is a bigger change.