Decouple error responses from exception

[lordrhodos]

This PR decouples the generation of the error responses from the OAuthServerException by moving the function to a new ExceptionResponseHandlerInterface with a default ExceptionResponseHandler implementation.

The handler gets injected into the authorization and resource server and is used via a ::generateHttpRespone proxy method.

This way required changes are minized while the libraries error responses can be easily overridden or adapted. This change is motivated by adding support for translated error response payloads in a second step.

Relates to #972, #884, #747

[strongholdmedia]

I find this idea an awesome one.