When Death Watch is used, the watcher will receive a Terminated(watched)
message when watched is terminated.
Terminated message can't be forwarded to another actor, since that actor
might not be watching the subject. Instead, if you need to forward Terminated
to another actor you should send the information in your own message.
param: actor the watched actor that terminated
param: existenceConfirmed is false when the Terminated message was not sent
directly from the watched actor, but derived from another source, such as
when watching a non-local ActorRef, which might not have been resolved
param: addressTerminated the Terminated message was derived from
that the remote node hosting the watched actor was detected as unreachable