code: func (v *Validator) AddError(field string, msg string) *ValidationResult {
  	result := &ValidationResult{
  		field:   field,
  		message: msg,
  	}
  	v.results = append(v.results, result)
  	return result
  }
docs: // AddError adds an error associated with field to the validator. msg
  // should typically be a user-readable sentence, such as "username
  // is required."