code: def handle_callback_exception(self, callback):
          
          if self._error_handler:
              self._error_handler(callback)
          else:
              app_log.error("Exception in callback %r", callback, exc_info=True)
docs: This method is called whenever a callback run by the `IOLoop`
          throws an exception.
  
          By default simply logs the exception as an error.  Subclasses
          may override this method to customize reporting of exceptions.
  
          The exception itself is not passed explicitly, but is available
          in `sys.exc_info`.