Default fallbacks should be media type-aware

The default fallbacks for unauthorized and forbidden requests should use content negotiation to produce a response with a content type adequate to the `Accept` request header.

At least the most common media types should be supported: `text/plain`, `text/html`, `application/json` and `application/xml`.

See [this discussion](https://github.com/mohiva/play-silhouette/issues/16#issuecomment-31771457).