Prepend error to document.title when form submission fails

Problem/Motivation

From comment #15 in #3258030: Text fields using CKEditor 5 do not get visual error indicator →

"the document title remains the same after the form submission failed due to a missing required field filled out properly or at all.
it might be helpful for especially screenreader users to update the document title that it shows something like (1 error) Create Article | Drupal 9 instead. that way a screenreader user has a direct feedback on page load knowing that something went wrong on form submission. another use case is for sighted users who have many tabs open. if one users gets distracted and pulled away from the computer for a while and when revisiting the browser with the updated document title it would be easily distinguishable and scanable which was the tab in question with an error.

*the suggestion about the pattern for prepending (x error(s)) to the document.title is from the form design patterns book by adam silver"

Steps to reproduce

1. Make sure that body field on basic page (or whatever content) /admin/structure/types/manage/page/fields/node.page.body
is set as required.
2. Edit content and fill in everything except the body field so the error will appear
3. Hit Save

Proposed resolution

Update document.title to prepend error like
(1 error) Create Article | Drupal 9

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet