UBI: fix and clean-up error paths in WL worker

This patch fixes the error path in the WL worker - in same cases
UBI oopses when 'goto out_error' happens and e1 or e2 are NULL.
This patch also cleans up the error paths a little. And I have
tested nearly all error paths in the WL worker.

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