provide flag to ensure Python thread shutdown

On Mark's laptop (Mac OS X/JRE 6), if a Thread.stop() signal arrived
while the thread was asleep, the thread wouldn't actually stop. The
sleep would interrupt, but the thread would continue running.

So, JESThread now includes a flag that is set to True when the thread
is stopped. Before and after JESDebugger sleeps, it checks the flag,
and raises ThreadDeath if one was not already raised by the .stop().
override exception message for ThreadDeath

java.lang.ThreadDeath is kind of a scary message to see when you click
the "Stop" button. So, now, instead of just having a message that gets
buried below the death-having traceback, JESExceptionRecord completely
overrides the dislay logic for ThreadDeath exceptions, to print out a
simple message saying that the program stopped.