swaybar: silence missing IconThemePath message

IconThemePath is not a standard property in XDG's StatusNotifierItem
specification, so missing this property should not be logged as an error.

This patch changes the log level to SWAY_DEBUG when swaybar queries the
value of IconThemePath so that swaybar won't log the returned message as
an error if IconThemePath does not exist.

Closes: swaywm#6092

swaybar/tray/item.c

@@ -118,8 +118,13 @@ static int get_property_callback(sd_bus_message *msg, void *data,
 
 	int ret;
 	if (sd_bus_message_is_method_error(msg, NULL)) {
-		sway_log(SWAY_ERROR, "%s %s: %s", sni->watcher_id, prop,
-				sd_bus_message_get_error(msg)->message);
+		const sd_bus_error *err = sd_bus_message_get_error(msg);
+		sway_log_importance_t log_lv = SWAY_ERROR;
+		if ((!strcmp(prop, "IconThemePath")) &&
+				(!strcmp(err->name, SD_BUS_ERROR_UNKNOWN_PROPERTY))) {
+			log_lv = SWAY_DEBUG;
+		}
+		sway_log(log_lv, "%s %s: %s", sni->watcher_id, prop, err->message);
 		ret = sd_bus_message_get_errno(msg);
 		goto cleanup;
 	}