[api] Make sure message_bus_queue is implemented

This causes message_bus_queue to raise an exception, if the event
subclass is not implementing it. This will make sure the method get's
implemented by any subclass of Event.

src/api/app/models/event/base.rb

@@ -80,6 +80,10 @@ def inherited(subclass)
         subclass.create_jobs(*create_jobs)
         subclass.receiver_roles(*receiver_roles)
       end
+
+      def message_bus_queue
+        raise NotImplementedError
+      end
     end
 
     # just for convenience