Issue #3520669 by alexpott: Change warning added in [#3517991] to an error

src/EntityUsageUpdater.php

@@ -851,7 +851,7 @@ class EntityUsageUpdater implements EntityUsageUpdaterInterface {
     foreach ($check_usages as $old_entity_type_id => $entity_updates) {
       foreach (\Drupal::entityTypeManager()->getStorage($old_entity_type_id)->loadMultiple($entity_updates) as $entity) {
         if (static::countUsages($entity) > 0) {
-          $context['results']['warnings'][] = t(
+          $context['results']['errors'][] = t(
             'There are still usages for %label (@url).',
             [
               '%label' => $entity->label() ?? $entity->getEntityTypeId() . ':' . $entity->id(),

tests/src/Kernel/EntityUsageUpdaterViolationTest.php

@@ -96,7 +96,10 @@ class EntityUsageUpdaterViolationTest extends EntityUsageUpdaterKernelTestBase {
 
     // Test messenger message.
     $messages = $this->container->get('messenger')->messagesByType('error');
-    $this->assertCount(1, $messages);
+    $this->assertCount(2, $messages);
+    // The first message is the one we are interested in. The second message
+    // comes from
+    // \Drupal\entity_usage_updater\EntityUsageUpdater::checkUsageAfterRemove().
     $this->assertInstanceOf(Markup::class, $messages[0]);
     $this->assertSame('Cannot save <em class="placeholder">entity_test</em> <em class="placeholder">3</em> because: This value should not be null.', (string) $messages[0]);