Fix: Fixed the use of possibly undefined variable in Jinja2 template.

lib/hawat/blueprints/design/templates/http_error.html

@@ -11,7 +11,7 @@
                 {{ error }}!
             </p>
             <hr>
-    {%- if exception.description %}
+    {%- if exception and exception.description %}
         <p class="text-center">
             {{ exception.description | safe }}
         </p>