R1:41a8837f78db

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…