BREAKING CHANGE: Fixes for unknown device errors

* If we can't send an invite due to unknown devices, abort the
   call.
 * Don't transition to the `invite_sent` state until the invite
   has actually sent.
 * Add a specific error code for failure due to unknown devices.
 * Don't send ICE candidate messages if the call has ended.
 * Add an `event` property to errors from `sendEvent` so that the
   caller can resend or cancel the event.

src/client.js

@@ -1127,6 +1127,9 @@ function _sendEvent(client, room, event, callback) {
         try {
             _updatePendingEventStatus(room, event, EventStatus.NOT_SENT);
             event.error = err;
+            // also put the event object on the error: the caller will need this
+            // to resend or cancel the event
+            err.event = event;
 
             if (callback) {
                 callback(err);