Problem/Motivation
Validation messages for errors in the block configuration forms of do not get displayed because of #2897377: $form['#attributes']['data-drupal-selector'] is set to a random value so can't be used for its intended purpose.
Proposed resolution
Set the $form['#id'] explicitly so the messages will be displayed.
Remaining tasks
Review
User interface changes
the error messages will show.
API changes
None
Data model changes
None