Return false for timed-out `isOnline` calls

There's an ask involved in resolving isOnline, which, though it seems
unlikely, could possibly time out. If this happens, we don't want that
AskTimeout exception to escape and blow things up. So, call a timeout
"false", and let the polling take care of the rest.
I'm wondering about the timing here; the apidoc for `ActorSystem.stop()` says that it is an asynchronous operation--there's no guarantee the supervisor is dead before you call `.isOnline`. Can we be sure that a timeout really happened here? The test would pass if the supervisor was still alive but Sirius was not initialized.