Make the Aphlict server more resilient.

Summary:
Currently, the Aphlict server will crash if invalid JSON data is POSTed 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. POSTed 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