Calling back with the underlying request error on failures

Hi, thanks for creating this library!  I'm suggesting a small error-handling change... right now, if the request library calls back with an error instead of with a request object, it hits the "no response received!" branch, which returns a generic message instead of the specific failure (which might not be on the Librato end -- could be a network connectivity issue, etc.)  request always calls back with either an error or a response, so the check for a missing response isn't really necessary if we check for an error.