Add retry button to 500 page

aleksis/core/templates/500.html

@@ -16,6 +16,10 @@
           {% endblocktrans %}
         </p>
         {% include "core/partials/admins_list.html" %}
+        <a href="javascript:window.location.reload()" class="btn green waves-effect waves-light">
+          <i class="material-icons left">refresh</i>
+          {% trans "Retry" %}
+        </a>
       </div>
     </div>
   </div>