Support customizable view EXCEPTION_HANDLER (#907)

Add `api_settings.EXCEPTION_HANDLER` to support custom error responses.

I still need to add a test for this, but I think this handles the usage discussed in #907 and [on the mailing list](https://groups.google.com/forum/#!msg/django-rest-framework/ndgmIU8kfOY/M_tMmAiPhp8J).