[project @ 2004-10-05 15:56:17 by simonmar]

[project @ 2004-10-05 15:56:17 by simonmar]
If we try to use a finalized handle, then throw a useful exception.
Before, the thread would just block forever on the MVar or get a
BlockedOnDeadMVar exception.

The fact that you can actually get into a situation where a finalized
handle can be accessed is arguably a bug itself, but at least now
we'll be able to quickly see when it is happening which should cut
down on debugging time.