fix(porch-store): log a genuine pool_count trap, not just 503 silently

- catch (e) nil could not distinguish a real store failure (e.g. a
  mod-by-zero from Pool { actors: [] }) from ordinary saturation
- print_err the trap message before answering 503, matching the same
  fix in idempotent.wo's pool_begin catch

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit c53ad583051abeeeb302301fc3d91073f0a15fcc)

docs/examples/porch/middleware/limiter.wo

@@ -34,7 +34,10 @@ pub class Limiter {
 
   fn before(mut req: Req) -> ?Resp {
     let key = limiter_key(self, req);
-    let v = try pool_count(self.pool, key, self.limit, self.window) catch (e) nil;
+    let v = try pool_count(self.pool, key, self.limit, self.window) catch (e) {
+      print_err("limiter: pool_count trapped: ${e.msg}");
+      nil
+    };
 
     if v == nil {
       let r = Resp { status: 503, headers: {}, body: "{\"error\":\"rate limiter saturated\"}" };