Content-negotation for error handlers

[ordnungswidrig]

Error responses might provide need a different set of media-types for the response, e.g. a resource the provides an image of types image/jpg and image/png might want to return a 401 unauthorized with the media types text/plain, text/html or application/json instead.

In addition liberators handler see a negotiated media type after successful content negotiation. That means that for handlers like handle-unauthorized or handle-service-not-available cannot provide a matching response without additional work.

[ordnungswidrig]

If implemented this will fix #69, #122, #82

[cbui]

I'm having a hard time understanding what the proposed solution is, and I read the other threads. Could you reexplain?

[ordnungswidrig]

@Christopher-Bui I propose to add another declaration in the resource definition for the media types of error responses and to negotiate the content type when necessary. As a default available-media-types et. al. would be used.

[ordnungswidrig]

This will also likely fix #76