Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/modules/system/templates/details.html.twig

@@ -5,6 +5,7 @@
  *
  * Available variables
  * - attributes: A list of HTML attributes for the details element.
+ * - errors: (optional) Any errors for this details element, may not be set.
  * - title: (optional) The title of the element, may not be set.
  * - description: (optional) The description of the element, may not be set.
  * - children: (optional) The children of the element, may not be set.
@@ -20,6 +21,12 @@
     <summary{{ summary_attributes }}>{{ title }}</summary>
   {%- endif -%}
 
+  {% if errors %}
+    <div>
+      {{ errors }}
+    </div>
+  {% endif %}
+
   {{ description }}
   {{ children }}
   {{ value }}