Responding with error in case of first process start failure.

After shared application port introducing, request queue in router was
removed and requests may stuck forever waiting for another process start.

src/nxt_http.h

@@ -169,6 +169,10 @@ struct nxt_http_request_s {
     nxt_http_peer_t                 *peer;
     nxt_buf_t                       *last;
 
+    nxt_queue_link_t                app_link;   /* nxt_app_t.ack_waiting_req */
+    nxt_event_engine_t              *engine;
+    nxt_work_t                      err_work;
+
     nxt_http_response_t             resp;
 
     nxt_http_status_t               status:16;