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/cdev.c b/drivers/mtd/ubi/cdev.c
index 9771e7f..12777da 100644
--- a/drivers/mtd/ubi/cdev.c
+++ b/drivers/mtd/ubi/cdev.c
@@ -377,7 +377,8 @@
 
 	err = ubi_more_update_data(ubi, vol->vol_id, buf, count);
 	if (err < 0) {
-		ubi_err("cannot write %zd bytes of update data", count);
+		ubi_err("cannot write %zd bytes of update data, error %d",
+			count, err);
 		return err;
 	}