Handler errors throw instead of returning styled divs

api and call-handler now use (error ...) for not-found and method
mismatch, so error boundaries catch them properly instead of
rendering error messages as page content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/handlers/dispatch.sx

@@ -6,10 +6,7 @@
       ((handler-key (str "handler:ex-" slug)) (hdef (env-get handler-key)))
       (if
         (nil? hdef)
-        (div
-          :class "p-8"
-          (h2 :class "text-rose-600 font-semibold" "Handler not found")
-          (p :class "text-stone-500" (str "No handler: " handler-key)))
+        (error (str "Handler not found: " handler-key))
         (call-handler hdef)))))
 
 (define
@@ -28,8 +25,7 @@
           (and
             (not (= handler-method "GET"))
             (not (= req-method handler-method)))
-          (div
-            :class "p-4 text-rose-600"
+          (error
             (str
               "Method not allowed: expected "
               handler-method