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_router.h

@@ -138,6 +138,7 @@ struct nxt_app_s {
     nxt_str_t              conf;
 
     nxt_atomic_t           use_count;
+    nxt_queue_t            ack_waiting_req; /* of nxt_http_request_t.app_link */
 
     nxt_app_joint_t        *joint;
     nxt_port_t             *shared_port;