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
ah, the other occurrence is here: 
https://github.com/symfony-cmf/MenuBundle/blob/master/DependencyInjection/CmfMenuExtension.php#L137-L139

now it seems either this or the other place are wrong about the package name (key-value-form-bundle vs key-value-bundle).

The setup instruction in the exception seems a bit much to me. I would rather do something like we have in menu bundle. If you insist, lets remove the version from the require (making composer install the latest stable) and say `and add %s to AppKernel`.