Description of problem:
When editing a role, Zanata should prevent the role renaming to existing role.

Version-Release number of selected component (if applicable):
Zanata version 2.3-SNAPSHOT (20130412-1235)

How reproducible:
Always

Steps to Reproduce:
1. Sign in as admin
2. Cick More->administration->Manage role
3. Create 2 new roles, 'ROLE1' and 'ROLE2', ROLE2 is a member of ROLE1
4. Click *Edit* on ROLE2
5. Rename ROLE2 into 'admin'
6. Save
  
Actual results:
ROLE2 disappeard, and admin now become member of "ROLE1", existing "member of" of admin is overwritten.

Expected results:
Error message: Role "admin" already exists.

Additional info: