Some generic and uninformative error messages

Under some circumstances, the error message reported is not very informative; try, for example, to

1. define a user or role schema with unique flag
2. create first user (or group) with certain value for such a schema
3. attempt to create another user (or group) with same value
4. error is raised (correctly) but message says "DataIntegrityViolation [The transaction has been rolled back. See the nested exceptions for details on the errors that occurred.]"

Besides this specific case, more tests should be added to check at least every case managed by RestServiceExceptionMapper.