code: func (f *Field) ErrorClass() string {
  	if f.Error != nil {
  		if errorClass, ok := f.viewArgs["ERROR_CLASS"]; ok {
  			return errorClass.(string)
  		}
  		return ErrorCSSClass
  	}
  	return ""
  }
docs: // ErrorClass returns ErrorCSSClass if this field has a validation error, else empty string.