UBI: add more prints

I hit those situations and found out lack of print messages. Add more prints
when erase problems occur.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/wl.c b/drivers/mtd/ubi/wl.c
index a5a9b8d..12b25e5 100644
--- a/drivers/mtd/ubi/wl.c
+++ b/drivers/mtd/ubi/wl.c
@@ -1093,6 +1093,7 @@
 		return err;
 	}
 
+	ubi_err("failed to erase PEB %d, error %d", pnum, err);
 	kfree(wl_wrk);
 	kmem_cache_free(wl_entries_slab, e);