magento/magento2#8114: "Save Block"-button on Add New Block silently ignores...

magento/magento2#8114: "Save Block"-button on Add New Block silently ignores clicks if the content is empty.

app/code/Magento/Ui/view/base/web/templates/content/content.html

@@ -17,3 +17,5 @@
         <span repeat="8"/>
     </div>
 </div>
+
+<label class="admin__field-error" if="error" attr="for: uid" text="error"/>