[8.x] Add ability to throw a custom validation exception

[stevebauman]

Description

This PR adds the ability to set a custom validation exception for manually created Validator instances via a setException() method.

The custom validation exception must extend the current ValidationException.

Purpose

This is super helpful for customizing the ValidationException with extra functionality that an application may need for specific validator instances.

The only other way of doing this right now (to my knowledge) is to extend the validator factory and modify the methods that throw the validation exceptions.

Example

class AppValidationException extends \Illuminate\Validation\ValidationException {}

// Somewhere in the application...

$validator = Validator::make($data, $rules, $messages);

$validator->setException(AppValidationException::class);

$validator->validate();

Notes

Let me know if you'd like anything adjusted or modified. No hard feelings on closure ❤️

Thanks for your time!

[driesvints]

Seems like there's some conflicts here and tests aren't running yet. Please rebase this and make sure all builds pass. Then mark as ready to review. Thanks.