net/nanocoap: don't abort server on recv error

sys/net/application_layer/nanocoap/sock.c

@@ -133,8 +133,7 @@ int nanocoap_server(sock_udp_ep_t *local, uint8_t *buf, size_t bufsize)
     while (1) {
         res = sock_udp_recv(&sock, buf, bufsize, -1, &remote);
         if (res < 0) {
-            DEBUG("error receiving UDP packet\n");
-            return -1;
+            DEBUG("error receiving UDP packet %d\n", (int)res);
         }
         else if (res > 0) {
             coap_pkt_t pkt;