Add "Debug" button to inspect the response of a failed AJAX request

Related to #118

less/lib/scaffolding.less

@@ -130,3 +130,10 @@ blockquote ol:last-child {
     position: fixed;
   }
 }
+
+.RequestErrorModal {
+  pre {
+    white-space: pre-wrap;
+    margin: 0;
+  }
+}