Add headers to REST error object

[Harrison-M]

Problem: Twitter provides rate limit information via custom HTTP headers on each request, but, as of commit 8fcea75, the promise interface no longer exposes the full response from Twitter, so this info is no longer available from 4xx or 5xx responses.

Solution: Attach the headers to the error in the same way that the response body and status code are attached. Since this is relevant metadata for managing Twitter requests, it makes sense to me to include this here, and it shouldn't be too much noise. I also considered attaching the full response to the error object, but that felt excessive. I'm open to discussion either way, though. Thanks for all your work on this!