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