Group security violation 11479

See #2369 and https://trac.openmicroscopy.org.uk/ome/ticket/11479

If Admin tries to change a 'read-annoate' group to 'private', this will fail if any links have been created between users' data (see #2369) and this PR handles the Exception thrown.

To test:
 - Pick or create a read-annotate group that has links between one user's data and another user's tags.
 - As Admin, edit the group, trying to set it to 'priavate'
 - Should see an appropriate warning that this may fail.
 - It should fail and you get a warning.

![screen shot 2014-05-13 at 00 04 48](https://cloud.githubusercontent.com/assets/900055/2951584/e8a843ec-da29-11e3-9977-5fd0d7e5b388.png)