code: func (e *APIError) Error() string {
  	var buf bytes.Buffer
  	fmt.Fprintf(&buf, "linebot: APIError %d ", e.Code)
  	if e.Response != nil {
  		fmt.Fprintf(&buf, "%s", e.Response.Message)
  		for _, d := range e.Response.Details {
  			fmt.Fprintf(&buf, "\n[%s] %s", d.Property, d.Message)
  		}
  	}
  	return buf.String()
  }
docs: // Error method