added error message for nonunique tagname

__Tasks:__
- [x] done
- [x] got feedback

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none

__Requires:__
- https://github.com/massiveart/husky/pull/402
- AdminBundle update