code: def invalid_config_error_message(action, key, val):
      
      if action in ('store_true', 'store_false'):
          return ("{0} is not a valid value for {1} option, "
                  "please specify a boolean value like yes/no, "
                  "true/false or 1/0 instead.").format(val, key)
  
      return ("{0} is not a valid value for {1} option, "
              "please specify a numerical value like 1/0 "
              "instead.").format(val, key)
docs: Returns a better error message when invalid configuration option
      is provided.