[project @ 2000-03-16 17:27:12 by simonmar]

[project @ 2000-03-16 17:27:12 by simonmar]
Raise an exception in any thread that is about to be garbage
collected.

	- threads blocked on MVars that are otherwise garbage
	  will receive a BlockedOnDeadMVar exception.

	- threads blocked on Black Holes must be in some kind
	  of deadlock, so we send them a NonTermination exception.