Show friendly error page when a service is unavailable

FragmentError now renders a 503 page naming which service is down
instead of a generic 500 error. Helps debug during deploys.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/browser/templates/_types/root/exceptions/error.html

@@ -2,7 +2,11 @@
 
 {% block error_summary %}
   <div>
-    WELL THIS IS EMBARASSING...
+    {% if service_name %}
+      The <b>{{ service_name }}</b> service is currently unavailable. It may be restarting.
+    {% else %}
+      WELL THIS IS EMBARASSING...
+    {% endif %}
   </div>
 {% endblock %}