Make the Aphlict server more resilient.

Summary:
Currently, the Aphlict server will crash if invalid JSON data is `POST`ed to it. I have fixed this to, instead, return a 400. Also made some minor formatting changes.

Ref T4324. Ref T5284. Also, modify the data structure that is passed around (i.e. `POST`ed to the Aphlict server and broadcast to the Aphlict clients) to include the subscribers. Initially, I figured that we shouldn't expose this information to the clients... however, it is necessary for T4324 that the `AphlictMaster` is able to route a notification to the appropriate clients.

Test Plan:
Making the following `curl` request: `curl --data "{" http://localhost:22281/`.

**Before**
```
sudo ./bin/aphlict debug
Starting Aphlict server in foreground...
Launching server:

    $ 'nodejs' '/usr/src/phabricator/src/applications/aphlict/management/../../../../support/aphlict/server/aphlict_server.js' --port='22280' --admin='22281' --host='localhost' --user='aphlict'

[Wed Jun 11 2014 17:07:51 GMT+0000 (UTC)] Started Server (PID 2033)
[Wed Jun 11 2014 17:07:55 GMT+0000 (UTC)]
<<< UNCAUGHT EXCEPTION! >>>

SyntaxError: Unexpected end of input
>>> Server exited!
```

**After**
(No output... the bad JSON is caught and a 400 is returned)

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4324, T5284

Differential Revision: https://secure.phabricator.com/D9480