keyIsInvalid(key, genericKey = MongoObject.makeKeyGeneric(key)) {
    if (Object.prototype.hasOwnProperty.call(this._deps, genericKey)) this._deps[genericKey].depend();

    return this._keyIsInvalid(key, genericKey);
  }
Returns, whether there is an error for a given key. Reactive.
@param key {string}
@param genericKey {string}
@return {boolean}