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/app/errors.py

@@ -9,6 +9,7 @@ from quart import (
 )
 
 from markupsafe import escape
+from shared.infrastructure.fragments import FragmentError as _FragmentError
 
 class AppError(ValueError):
     """
@@ -107,6 +108,23 @@ def errors(app):
         )
         return await make_response(html, status)
 
+    @app.errorhandler(_FragmentError)
+    async def fragment_error(e):
+        current_app.logger.error("FragmentError %s", _info(e))
+        msg = str(e)
+        # Extract service name from "Fragment account/auth-menu failed: ..."
+        service = msg.split("/")[0].replace("Fragment ", "") if "/" in msg else "unknown"
+        if request.headers.get("HX-Request") == "true":
+            return await make_response(
+                f"<p class='text-sm text-red-600'>Service <b>{escape(service)}</b> is unavailable.</p>",
+                503,
+            )
+        html = await render_template(
+            "_types/root/exceptions/error.html",
+            service_name=service,
+        )
+        return await make_response(html, 503)
+
     @app.errorhandler(Exception)
     async def error(e):
         current_app.logger.exception("Exception %s", _info(e))

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 %}