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