code: func ValidateJsonString(v interface{}, k string) (ws []string, errors []error) {
  	if _, err := structure.NormalizeJsonString(v); err != nil {
  		errors = append(errors, fmt.Errorf("%q contains an invalid JSON: %s", k, err))
  	}
  	return
  }
docs: // ValidateJsonString is a SchemaValidateFunc which tests to make sure the
  // supplied string is valid JSON.