How do I get Tornado to write exceptions to a log file?

I have a Flask application, which uses Tornado as the web server. On the live server, something is going wrong and I get the error message "500 Internal Server Error".

I want to know what is going wrong, and I'd like the text of all uncaught exceptions to go to the log file. How do I do this?

My code to start up Tornado looks like this:

def doLogging():
    """ make Tornado do logging """
    import tornado.options
    import tornado.log
    logger = logging.getLogger("ewa")
    pathToHere = os.path.dirname(os.path.realpath(__file__))
    logPan = butil.join(pathToHere, "../logs/ewa.log")
    fh = logging.FileHandler(logPan)
    fh.setLevel(logging.DEBUG)
    formatter = logging.Formatter("%(asctime)s:%(name)s:"
        "%(levelname)s:%(message)s")
    fh.setFormatter(formatter)
    logger.addHandler(fh)

    tornado.log.enable_pretty_logging(None, logger)

    logger.info("===== web server started =====")
    logger.warn("(a warning message)")
    logger.error("(an error message)")
    logger.critical("(a critical message)")

if __name__ == "__main__":
    doLogging()
    container = wsgi.WSGIContainer(main.app)
    http_server = httpserver.HTTPServer(container)
    http_server.listen(8210)
    ioloop.IOLoop.instance().start()

This correctly creates the log file, but uncaught exceptions aren't being logged.