Element validation errors are not visible in the widget setting form

Problem/Motivation

The problem is that the validation errors are not visible in the maxlength widget configuration form.
The issue originates from the fact that the widget setting form is AJAX and the error message is not printed on the top of the page unless we refresh the page.

The tests are failing in MaxLengthCustomWidgetTest due to this problem.

Remaining tasks

• ❌ Get maintainer approval for the proposed resolution
• ❌ Implement resolution
• ❌ Write test coverage (contact a maintainer if you need help)
• ❌ Maintainer review via the UI
• ❌ Maintainer Code Review #1
• ❌ Maintainer code review #2
• ❌ Merge into dev branch, with credit to author and participants

User interface changes

API changes

Data model changes