ScopedThread's join() method should throw if an exception was thrown in the thread

Almost all tests using ScopedThreads call join() on the thread, but few if any call hasFailed(). Without checking the result of hasFailed() on the thread, it's possible that an uncaught exception was thrown from the thread but the test can still pass. This may be hiding existing test failures.

If we don't do this, we should at least audit all users of ScopedThread and make sure they're handing errors from within the thread appropriately.