Improve expection message

Closes #3449851

core/lib/Drupal/Core/Config/Plugin/Validation/Constraint/LangcodeRequiredIfTranslatableValuesConstraintValidator.php

@@ -22,8 +22,12 @@ public function validate(mixed $value, Constraint $constraint): void {
 
     $mapping = $this->context->getObject();
     assert($mapping instanceof Mapping);
-    if ($mapping !== $this->context->getRoot()) {
-      throw new LogicException('The LangcodeRequiredIfTranslatableValues constraint can only operate on the root object being validated.');
+    $root = $this->context->getRoot();
+    if ($mapping !== $root) {
+      throw new LogicException(sprintf(
+        'The LangcodeRequiredIfTranslatableValues constraint is applied to \'%s\'. This constraint can only operate on the root object being validated.',
+        $root->getName() . '::' . $mapping->getName()
+      ));
     }
 
     assert(in_array('langcode', $mapping->getValidKeys(), TRUE));