Exclude map/reduce errors from main log.

Because if the errors are common, they can easily spam our log.

Change-Id: Ief8e2c0b2cb6e8f6319c8dea72dde184a6254f49
Reviewed-on: http://review.couchbase.org/37131
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>