docs: def handle_url_error(error, endpoint, values):
      """
      Intercept BuildErrors of url_for() using flasks build_error_handler API
      """
      url = overlay_url_for(endpoint, **values)
      if url is None:
          exc_type, exc_value, tb = sys.exc_info()
          if exc_value is error:
              reraise(exc_type, exc_value, tb)
          else:
              raise error
      # url_for will use this result, instead of raising BuildError.
      return url