[JENKINS-65017] Prevent bugs in form validation from messing up the entire page

[daniel-beck]

A simple attempt to prevent NPEs and similar problems in form validation URLs from blowing up forms.

Previously, an entire Jenkins page would be shown below the affected form field, including the Logging ID that replaced stack traces, so we lose a bit of functionality here, but it's just a lot nicer looking (and could be further iterated on, e.g. by more flexibly serving 500 responses based on request type).

Manual testing: Edit any validated form field's checkurl to an invalid one, and observe:

Before

(Current LTS)

After

(This PR)

Proposed changelog entries

• Do not render full error responses in case of internal errors when validating fields in configuration forms.

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

@mention

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).

[oleg-nenashev]

Reloading the page is probably not the best advice. Some unsaved form data may be lost if a user follows this advice.

I also wonder whether it is possible to capture logging ID of the unhandled exception from the original page so that the administrator can figure out what has happened.

[daniel-beck]

Reloading the page is probably not the best advice. Some unsaved form data may be lost if a user follows this advice.

The dialog with the reminder appears on reload attempts. A potential cause for this error is session expiration, in which case all your data is lost anyway.

I also wonder whether it is possible to capture logging ID of the unhandled exception from the original page so that the administrator can figure out what has happened.

If it's a 500 page with logging ID; probably, I see that as future enhancement (I've done something similar with the Apply response before I think).

[timja]

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

Thanks!