Show error message if group permission downgrade fails

Fixes: https://trac.openmicroscopy.org.uk/ome/ticket/12272
With this PR the user will see an error message if the downgrade of group permission to read-only fails (now: fails silently).

Test:

Create a read-annotate group, add some data as owner, downgrade to read-only: Should work
Upgrade to read-annotate again; add some annotations as a non-owner user, try to downgrade to read-only again: Should fail, and you should see an error message.