Fix a bug where with-watchers wasn't bubbling the willTransition event

The impact was the application error was no longer being nulled out,
causing the application error to continue to be shown after
transitioning.

This never happened in apps since it's not possible to transition away
from the error screen.

ui/app/mixins/with-watchers.js

@@ -33,6 +33,9 @@ export default Mixin.create(WithVisibilityDetection, {
   actions: {
     willTransition() {
       this.cancelAllWatchers();
+
+      // Bubble the action up to the application route
+      return true;
     },
   },
 });