code: func WithMessagef(err error, format string, args ...interface{}) error {
  	if err == nil {
  		return nil
  	}
  	return &withMessage{
  		cause: err,
  		msg:   fmt.Sprintf(format, args...),
  	}
  }
docs: // WithMessagef annotates err with the format specifier.
  // If err is nil, WithMessagef returns nil.