fix: actually catch the error code from MoveFileEx (#3753)

A user on Zulip [reported seeing an error code of "no
error"](https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/.60lake.20update.60.20broken.20on.20Windows.20.28.3F.29/near/429134334)
here.
1 file changed