throw exception for non existing burgov bundle

 in combination with activated admin extension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #165 
| License       | MIT
| Doc PR        | need to be written
as does menubundle. its wrong over there.

i will sync it to what we end up with here. what do you think about the wording question?