Make HTTP errors returned from the Aphlict server more specific

Summary: Ref T5651. Currently, the Aphlict server returns either 200 OKAY or 400 Bad Request. We could return more specific errors in some cases and this may assist with debugging.

Test Plan:
Sent myself a test notification at /notification/status/ and saw the Aphlict server process the request (running in debug mode). Also poked around with curl:

> curl http://localhost:22281/
405 Method Not Allowed

> curl http://localhost:22281/ -d ""
400 Bad Request

> curl http://localhost:22281/foobar/
404 Not Found

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5651

Differential Revision: https://secure.phabricator.com/D9967