state: helpful error message if state GUID cannot be found

So far, if the user didn't specify a valid GPT UID, state initialization
failed with the not so helpful error message:

| ERROR: state state.of: probe failed: Invalid argument

Improve this by also providing a helpful error message.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20260113132444.3808198-1-ejo@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Author: ejoerns

common/state/state.c

@@ -651,6 +651,7 @@ struct state *state_new_from_node(struct device_node *node, bool readonly)
 	if (cdev_is_block_disk(cdev)) {
 		cdev = cdev_find_child_by_gpt_typeuuid(cdev, &barebox_state_partition_guid);
 		if (IS_ERR(cdev)) {
+			dev_err(&state->dev, "cannot find backend GPT partition by PartitionTypeGUID\n");
 			ret = -EINVAL;
 			goto out_release_state;
 		}