Add ability for 404 page to show error description.

realms/templates/errors/404.html

@@ -2,5 +2,8 @@
 {% block body %}
 
   <h1>Page Not Found</h1>
+  {% if error is defined %}
+  <h4>{{ error.description }}</h4>
+  {% endif %}
 
-{% endblock %}
+{% endblock %}