ValidationErrorBlock.php in Drupal 10        
                          
                  
                        
  
  
  
  
File
  core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/ValidationErrorBlock.php
  
    View source  
  <?php
namespace Drupal\settings_tray_test\Plugin\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
class ValidationErrorBlock extends BlockBase {
  
  public function build() {
    return [
      '#markup' => '<span>If I had more time this would be very witty :(.</span>',
    ];
  }
  
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::validateConfigurationForm($form, $form_state);
    $form_state
      ->setError($form['label'], 'Sorry system error. Please save again.');
  }
}