wrap ns_client_error() for unit testing

When testing, the client object doesn't have a proper
netmgr handle, so ns_client_error() needs to be a no-op.

(cherry picked from commit ae37ef45ff)

tests/ns/netmgr_wrap.c

@@ -97,3 +97,9 @@ isc_nmhandle_detach(isc_nmhandle_t **handlep) {
 
 	return;
 }
+
+void
+ns_client_error(ns_client_t *client ISC_ATTR_UNUSED,
+		isc_result_t result ISC_ATTR_UNUSED) {
+	return;
+}