Laravel FormRequest - Handling Failure Response

If I inject the FormRequest into the Controller method, such as:

public function createTask(CreateTaskRequest $request)
{
    // ...
}

I can validate all the data inside the CreateTaskRequest as usual using the rules() method. But how can I handle the response myself when the validation fails? There are cases for our API where I want to return an XML response, so I need some sort of way to access the error bag and output all the errors in the XML response.

via Chebli Mohamed