[10.x] Use ValidationException class from Validator Property

[a-h-abid]

Currently, the Validator class can take custom exception using setException method. So one can set custom ValiadtorException class as need and that will be used on validation error.

However in files FormRequest.php and ValidatesWhenResolvedTrait.php, the ValidationException class is hardcoded inside method failedValidation(). So if we use Form Requests, then it won't use the custom ValidationException class.

This PR makes it possible to use ValidationException class from the Validator class's $exception property. And it also makes possible to use custom exception fully throughout app.

How one can use custom ValidationException

By adding code like below in boot method of AppServiceProvider, you can use a custom validation exception.

\Illuminate\Support\Facades\Validator::resolver(function ($translator, $data, $rules, $messages, $customAttributes) {
    $validator = new \Illuminate\Validation\Validator($translator, $data, $rules, $messages, $customAttributes);
    $validator->setException(MyValidationException::class);
    return $validator;
});