Support `invalid_token` of RFC 6750

[jeremyb]

Hi,

Do you plan to support some part of the RFC 6750 "The OAuth 2.0 Authorization Framework: Bearer Token Usage"?
On my current project, I would like to implement the OAuth Extensions Error Registration part, especially the invalid_type error value.

For now I've overwritten the verifyAccessToken method of OAuth2\OAuth2 to handle this error type, but it should be interesting to add this support to the lib itself, or at least to make more flexible the convertion from an Exception to a Response...

I would be interested to contribute to this feature, but I'm a bit afraid of the BC breaks that can be related to these changes.