#727 Fail on timeout displays stack of stuck thread

New class: ExceptionWithThread.java (subclass of Exception that keeps information about some other relevant thread).  Changed: FailOnTimeout.java (starts test in a new ThreadGroup; on timeout, looks at running threads in ThreadGroup to see which one it thinks got stuck, creates an ExceptionWithThread if it finds one); Failure.java (displays stack for the "relevant thread" if it handles an ExceptionWithThread)