Additional error info on the HTTP 500 error page

Additional error info (text of the exception thrown by low level C++
code) is shown inside a text box of the same style as used for the
advice on the 404 error page (we either need to change the name of the
CSS style, or introduce a separate style for this piece of information).

static/templates/sexy500.html

@@ -22,5 +22,10 @@
       <p>{{PAGE_TEXT}}</p>
       <p><code>{{url_path}}</code></p>
     </section>
+{{#error}}
+    <section class="advice">
+      <p>{{error}}</p>
+    </section>
+{{/error}}
   </body>
 </html>