UBIFS: supress false error messages

Commit ab51afe05273741f72383529ef488aa1ea598ec6 was a good clean-up, but
it introduced a regression - now UBIFS prints scary error messages during
recovery on all corrupted nodes, even though the corruptions are expected
(due to a power cut). This patch fixes the issue.

Additionally fix a typo in a commentary introduced by the same commit.

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