feat(errors): allow raising Request Entity Too Large error

This patch adds the ability to now raise Request Entity Too Large error
directly, instead of just setting the status code on the Response
object.
I just realized there is a bug in the retry-after implementation used by HTTPServiceUnavailable. It needs to check whether *retry_after* is an instance of `datetime.datetime`, and if so, use *falcon.util.dt_to_http(...)* to serialize it to a string.

If you could fix it here, and then do me a favor by submitting a separate PR to fix it for HTTP 503, that would be super awesome. Thanks!