[Form] Throw an exception if the post_max_size has been reached.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11729
| License       | MIT
| Doc PR        | n/a
1. the if part should be in an `elseif` part.
1. I would not throw an exception, but I would add a FormError on the form.