[JENKINS-64429] Fix registerValidator breaking error messages

[fqueiruga]

See JENKINS-64429.

Changes to the registerValidator function on 2.264 (tables-to-divs) introduced a bug where server-side validations would corrupt the .validation-error-area internal HTML structure. This caused regexp validators to break and to fail to show messages.

Proposed changelog entries

• JENKINS-64429, Fixed error where server-side form validation would break validation on the client-side

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@timja
@amuniz
@jtnord

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[jtnord]

sure if it works :)

[amuniz]

Tested locally, it's fixing the issue.

Also fixing JENKINS-64651 - which is the same thing.

[MRamonLeon]

Great!

[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!