dns: ensure that update_dns() always returns a GError on failure

Callers may expect this, so make sure we do it.

src/NetworkManagerUtils.h

@@ -49,7 +49,7 @@ nm_utils_ip6_route_metric_normalize (guint32 metric)
 	return metric ? metric : 1024 /*NM_PLATFORM_ROUTE_METRIC_DEFAULT_IP6*/;
 }
 
-int nm_spawn_process (const char *args);
+int nm_spawn_process (const char *args, GError **error);
 
 int nm_utils_modprobe (GError **error, const char *arg1, ...) G_GNUC_NULL_TERMINATED;