FOSRest integration: Form errors format

```php

/**
 * @ApiDoc(output={"class" = "FooBund\Form\UserType", "form_errors" = true})
 */
```

Documents the output of https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/2-the-view-layer.md#forms-and-views

This will be typically used with the Swagger integration, specifically when defining an alternate response model for validation errors:

```php

/**
 * @ApiDoc(resonseMap={
 *     400 = {"class" = "FooBund\Form\UserType", "form_errors" = true}
 *  })
 */
```

### TODO

* [x] Implementation
* [x] Unit tests