Disallow attempting to send command from an actor's broadcast command

When sending an command to another actor with, e.g. self.actor.cmdr.call(), it is best to declare which command we are executing, with .call(forUserCmd=. Turn out that the hub rejects command send from an actor's broadcast command. Not positive that is the right thing to do, but it does feel right.

Two problems, though. One is that tron silently drops the command very early. The other is that the actorcore library allows such commands to be sent. This ticket addresses the second point: raise an exception when such commands are attempted.