Seperated 'group permission update' from 'group udpate'

This was necessary in order to be able to show a error message
if a permission downgrade fails
Just made some 'cosmetic' changes and comments
Removed some unnecessary code
Merge pull request #2830 from dominikl/rebased/develop/permission_downgrade_errmsg_3

Show error message if group permission downgrade fails (rebased onto develop)