code: public InternalServerErrorException unknownAction(String path, Enum<?> action)
      {
          // Error message.
          String message = String.format("Authlete %s API returned an unknown action: %s", path, action);
  
          // 500 Internal Server Error
          return internalServerError(message, null);
      }
docs: Create an {@link InternalServerErrorException} instance to indicate
  that the value of {@code action} parameter contained in a response
  from an Authlete API is unknown.