[FrameworkBundle] Improved controller class error

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/11304
| License       | MIT
| Doc PR        | 
Yes, I have a similar problem as it's not much clear what ```$route``` actually means. But I'm not sure if there is  better solution for providing route name in exception message.

@fabpot Any ideas?