[Form] Add a form error if post_max_size has been reached.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11729
| License       | MIT
| Doc PR        | n/a

This is my new PR following @lyrixx comments.
the form needs to be marked as submitted in this case though, so that the code can know that a submission ended up with errors rather than thinking it is not submitted (and so it is the initial display)