net/nanocoap: add debug message for server

sys/net/application_layer/nanocoap/sock.c

@@ -145,6 +145,9 @@ int nanocoap_server(sock_udp_ep_t *local, uint8_t *buf, size_t bufsize)
             if ((res = coap_handle_req(&pkt, buf, bufsize)) > 0) {
                 res = sock_udp_send(&sock, buf, res, &remote);
             }
+            else {
+                DEBUG("error handling request %d\n", (int)res);
+            }
         }
     }