Add required parameter to WaitUntilEmpty

You catch every exception, and re-raise it as an ActorException anyway. The type of exception isn't a core feature here. If there were a typo, then the unit test that checks for successful execution would catch it.

I'll change this to catch the exact exception as you suggest, but I think that's a wrong test. Changing the type of exception later on should *not* fail a unit test.