notification: Don't print failure to load backend in UI

This isn't actually helpful information to users generally

Closes #2152
Closes #2684

src/fe-gtk/plugin-notification.c

@@ -218,7 +218,7 @@ notification_plugin_init (hexchat_plugin *plugin_handle, char **plugin_name, cha
 	if (!notification_backend_init (&error))
 	{
 		if (error)
-			hexchat_printf(plugin_handle, "Failed loading notification plugin: %s\n", error);
+			g_debug("Failed loading notification plugin: %s\n", error);
 		return 0;
 	}