static async getMessage(data) {
    return `Please configure an error message for "${this.name}" by overriding "static async getMessage()"`;
  }
Called inside Validator.prototype._getMessage (see explanation).
@example
```js
class MyValidator extends Validator {
  static async getMessage() {
    return 'lowest prio, defined globally by Validator author'
  }
}
// globally overridden
MyValidator.getMessage = async() => 'overrides already configured message';
```
@overridable
@param {Partial<FeedbackMessageData>} [data]
@returns {Promise<string|Element>}