Return appropriate HTTP status codes on error

[obskyr]

This pull request makes sure errors actually return non-OK HTTP status codes. Before, all requests (that didn't cause an actual uncaught exception serverside) returned 200. With this change, they instead return what they should according to the documentation, and as for the undocumented ones according to common sense.

With this pull request, the errors are mapped as following:

• Error.NONE - 200 OK
• Error.INVALID_CLIENT - 400 Bad request
• Error.INVALID_SERVICE - 400 Bad request
• Error.INVALID_SECRET - 400 Bad request
• Error.DUPLICATE_LISTEN - 409 Conflict
• Error.RATE_TOOFAST - 429 Too many requests
• Error.SERVICE_NOTFOUND - 404 Not found
• Error.ARGUMENT_MISSING - 400 Bad request
• Error.INVALID_PUBKEY - 400 Bad request (Though this error is never used.)
• Error.CONNECTION_CLOSING - 499 Client closed request (I wasn't sure about this one either, since it also goes unused.)
• Error.NO_CHANGES - 400 Bad request

Pretty nice, eh? This also resolves #17.

[Mechazawa]

Nice, I just need to make sure this is reflected in the docs from now on.

[Mechazawa]

Error.CONNECTION_CLOSING WAS used when websockets were still in the API instead of a connector. I should remove it some time.