UBI: improve error messages

Always print error code with error messages, sometimes it is
extremely helpful info.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/vmt.c b/drivers/mtd/ubi/vmt.c
index 6609c31..787ce9e 100644
--- a/drivers/mtd/ubi/vmt.c
+++ b/drivers/mtd/ubi/vmt.c
@@ -305,7 +305,7 @@
 	dev = MKDEV(MAJOR(ubi->cdev.dev), vol_id + 1);
 	err = cdev_add(&vol->cdev, dev, 1);
 	if (err) {
-		ubi_err("cannot add character device for volume %d", vol_id);
+		ubi_err("cannot add character device");
 		goto out_mapping;
 	}
 
@@ -319,8 +319,10 @@
 	vol->dev.class = ubi_class;
 	sprintf(&vol->dev.bus_id[0], "%s_%d", ubi->ubi_name, vol->vol_id);
 	err = device_register(&vol->dev);
-	if (err)
+	if (err) {
+		ubi_err("cannot register device");
 		goto out_gluebi;
+	}
 
 	err = volume_sysfs_init(ubi, vol);
 	if (err)
@@ -364,6 +366,7 @@
 out_unlock:
 	spin_unlock(&ubi->volumes_lock);
 	kfree(vol);
+	ubi_err("cannot create volume %d, error %d", vol_id, err);
 	return err;
 
 	/*
@@ -380,6 +383,7 @@
 	ubi->volumes[vol_id] = NULL;
 	spin_unlock(&ubi->volumes_lock);
 	volume_sysfs_close(vol);
+	ubi_err("cannot create volume %d, error %d", vol_id, err);
 	return err;
 }
 
@@ -591,7 +595,8 @@
 	dev = MKDEV(MAJOR(ubi->cdev.dev), vol->vol_id + 1);
 	err = cdev_add(&vol->cdev, dev, 1);
 	if (err) {
-		ubi_err("cannot add character device for volume %d", vol_id);
+		ubi_err("cannot add character device for volume %d, error %d",
+			vol_id, err);
 		return err;
 	}