Add image to 404.html

doc.zih.tu-dresden.de/tud_theme/404.html

@@ -2,6 +2,8 @@
 
 {% block content %}
 
+<center><img src="/assets/images/sad_smiley.png" alt="Sad smiley - page not found" width="200" height="200"/></center>
+
 <h3 style="text-align:center">The page you requested could not be found.</h3>
 <h3 style="text-align:center">Probably it has been moved, please use the search bar.</h3>