code: func (err *ErrSession) Error() string {
  	if err.Err == nil {
  		return fmt.Sprintf("%s (%s)", stateTexts[err.State], err.Type)
  	}
  	return fmt.Sprintf("%s: %s (%s)", stateTexts[err.State], err.Err, err.Type)
  }
docs: // Error implements the buildin error interface.