ActorSystem always succeeds

[pjz]

In writing some more general Actors and interfaces to interact with them, it would be nice to have a way to say "get me the Actorsystem if one's running, but don't start one up" so I could throw nicer errors in that case, but currently there's no way to do that, apart from making my own actorsystem singleton, which seems to kind of miss the point. Could some way to do this be added?