UBI: handle write errors in WL worker

When a PEB is moved and a write error happens, UBI switches
to R/O mode, which is wrong, because we just copy the data
and may select a different PEB and re-try this. This patch
fixes WL worker's behavior.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>