getErrorForKey(key, genericKey = MongoObject.makeKeyGeneric(key)) {
    const errors = this._validationErrors;
    const errorForKey = errors.find((error) => error.name === key);
    if (errorForKey) return errorForKey;

    return errors.find((error) => error.name === genericKey);
  }
Returns a validation error for a given key.
@param key {string} the key of the field to access errors for
@param genericKey {string} generic version of the key, you usually don't need
 to explcitly call this. If you do, you need to wrap it using `MongoObject.makeKeyGeneric`
@return {ValidationError|undefined}