Crash on database connection failure

[atrigent]

There's not much the data store API can do without a database connection, and some connection errors (authentication failure, for example) are pretty much unrecoverable. For the ones that are recoverable, we can rely on Heroku's automatic restarting of the app.

[atrigent]

Also: without change e3893cb, some types of connection errors (if I disable my wifi, for example) cause the app to crash with this error:

14:32:59 web.1  | events.js:72
14:32:59 web.1  |         throw er; // Unhandled 'error' event
14:32:59 web.1  |               ^
14:32:59 web.1  | Error: failed to connect to [****.mongolab.com:****]
14:32:59 web.1  |     at null.<anonymous> (/home/atrigent/programming/finalsclub/MIT-Annotation-Data-Store/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:540:74)
14:32:59 web.1  |     at EventEmitter.emit (events.js:106:17)
14:32:59 web.1  |     at null.<anonymous> (/home/atrigent/programming/finalsclub/MIT-Annotation-Data-Store/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:140:15)
14:32:59 web.1  |     at EventEmitter.emit (events.js:98:17)
14:32:59 web.1  |     at Socket.<anonymous> (/home/atrigent/programming/finalsclub/MIT-Annotation-Data-Store/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection.js:478:10)
14:32:59 web.1  | exited with code 8
14:32:59 system | sending SIGTERM to all processes
14:32:59 web.1  |     at Socket.EventEmitter.emit (events.js:95:17)
14:32:59        |     at net.js:833:16
14:32:59        |     at process._tickCallback (node.js:415:13)

Authentication errors, however, do not cause this crash, and there is in fact no indication that they occurred. I'm not sure why. With the above mentioned commit, all connection errors simply produce a console error. This ticket is about making them all crash the app.