nit: although not linted, and not previously used in this file, I believe all of the testing infrastructure uses braces on the same lines for else statements. Sorry to be a pain, but I would prefer to update the style to match the style generally used in the rest of the marionette-* modules.

See: https://github.com/mozilla-b2g/marionette-js-client/blob/master/lib/marionette/client.js#L1301