Fix cms form reset on failed ajax form submission

[hype09]

1. Why is this change necessary?

In #3753, I inadvertently introduced the bug described in #4422.

2. What does this change do, exactly?

Instead of always clearing the form after the content is submitted, we only clear it when the submission was actually successful.

3. Describe each step to reproduce the issue or behaviour.

• Go to the contact form.
• Enter all the information.
• Ensure the server-side validation fails, e.g. by mis-typing the captcha.
• The form is being reset (which it shouldn't be).

4. Please link to the relevant issues (if any).

• Original bug: CMS form data is not cleared from local storage after form is submitted #3719
• Original bugfix: NEXT-00000 - Fix clearing of form-preserver storage on cms-form submit #3753
• Introduced bug: FormCmsHandler plugin resets form even with response type danger and info #4422

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written tests and verified that they fail without my change
• I have created a changelog file with all necessary information about my changes
• [ ] I have written or adjusted the documentation according to my changes
• [ ] This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[tobiasberge]

Hey @hype09,

sorry for our delayed review. I was able to reproduce the problem and your solution solves it. 👍

Thank you for your fix. 🎉

We have now imported your PR and will process it soon.

[tobiasberge]

Hi @hype09 your changes were merged to trunk: 9552099

Thank you for your contribution.