Fix racy 'matrixclient retrying' test

when a message send fails, the promise returned by `sendMessage` is
rejected. Until we switched to bluebird, the rejection was happily being
swallowed, but with bluebird, there's a better chance of the unhandled
rejection being caught by the runtime and mocha and failing the test.

src/scheduler.js

@@ -83,6 +83,8 @@ MatrixScheduler.prototype.removeEventFromQueue = function(event) {
     let removed = false;
     utils.removeElement(this._queues[name], function(element) {
         if (element.event.getId() === event.getId()) {
+            // XXX we should probably reject the promise?
+            // https://github.com/matrix-org/matrix-js-sdk/issues/496
             removed = true;
             return true;
         }