Return the locator#ready promise from server#ready

Right now, if the `locator.ready` routine throws an error it will be handed to the `app.yui.ready` callback. However, if an error occurs inside the `app.yui.ready` callback, that error is swallowed by ypromise and a very unhelpful error is shown: `Promise rejected but no error handlers were registered to it`. This patch simply returns the ypromise from `app.yui.ready` so we might attach an error handler to it.