Add support for (multiple) error bags

[claudiodekker]

Added

• Support for multiple error bags
• Prevent passing unexpected values.
• Extra testing for 'default' (no-errors) scenario
• EXTRA / SUGGESTED: Response::withErrors(..).

Fixed

• Use ViewErrorBag instead of MessageBag (it works due to proxy-calls, but it's not really intended to be used that way)

Remaining / newly found issues

• When an actual ValidationException gets thrown, error messages won't get formatted correctly due to the way the ValidationException is defined/handled by the ExceptionHandler. Here's a failing test that demonstrates the issue.

There's a few solutions to this that I can tell, and neither of them aren't really pretty:

• Extend the Application's ExceptionHandler, then swap it out in the container. This will allow us to intercept the ValidationException and re-format the messages.

• Swap out the Validator for our own, so we can throw a custom ValidationException.

• Tell the user to add a call to their Application's ExceptionHandler, so we don't have to override/extend any core-logic (less maintenance, more work for the end-user)

[reinink]

Thanks so much for your help here @claudiodekker! I've simplified things a little, but the concept was right. 👌