Invalid forms should return a HTTP 422 code

[Jupi007]

Q	A
Bug?	yes
New Feature?	yes
Bundle Version	2.5
Sulu Version	2.5
Browser Version	N/A

When a form isn't valid, Turbo expects a HTTP 422 code to be returned.
Otherwise form errors aren't handled correctly.

Actual Behavior

This bundle always returns a HTTP 200 code regardless of form errors.

Expected Behavior

Submit an invalid form should return a HTTP 422 when given data isn't valid.

Steps to Reproduce

Using Hotwire Turbo and this bundle, submit a form with invalid value and see the problem: no error displayed and no redirection.

Possible Solutions

Add a kernel.response event listener, and return a http 422 response if an invalid form is returned.
I'm working on a PR.