Fallback SBD Messaging Management, Events, and Testing

[neonphog]

The only path to entering sbd-fallback-messaging-mode is that we get an error establishing a webrtc connection or sending the first webrtc message.

Write tests that ensure the following:

1. if the first message fails to send, it is sent again over the fallback messaging method
2. if we receive a fallback message (and have not successfully sent one of our own over webrtc) we also switch to fallback, aborting trying to set up webrtc
3. a "Disconnected" event is NOT emitted when we switch to fallback mode
4. if we get any kind of error after the first successful send of a message over webrtc, we do NOT enter fallback mode, but instead the whole connection is closed and a "Disconnected" event is emitted