Reviewed by Mihai Parparita.

        Fix a small race condition that could cause the unit tests for
        new-run-webkit-tests to fail in a multithreaded environment.
        There was a small window in the code where one thread would
        check to see if a thread had raised an exception, do something,
        and then check to see if the thread was alive. If the other
        thread raised an exception and exited in between these two
        steps, the exception would be lost. Flipping the order of the
        two checks fixes the race, since we will now always check for an
        exception on the thread before exiting.

        https://bugs.webkit.org/show_bug.cgi?id=51579

        * Scripts/webkitpy/layout_tests/layout_package/message_broker.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76073 268f45cc-cd09-0410-ab3c-d52691b4dbfc