code: func GetErrorBody(err error) string {
  	rv := errors.GetData(err, errorBody)
  	message, ok := rv.(string)
  	if !ok {
  		return errors.GetMessage(err)
  	}
  	class := errors.GetClass(err)
  	if class == nil {
  		return message
  	}
  	return fmt.Sprintf("%s: %s", class.String(), message)
  }
docs: // GetErrorBody will return the user-visible error message given an error.
  // The message will be determined by errors.GetMessage() unless the error class
  // has an error body overridden by OverrideErrorBody.