for MB-7230: state why access log was seen corrupt

Basic debugging whenever we hit a read error on
scanning the access log

Change-Id: I4b02a9484e7d0b56c0691e062717729d9d7ed4d5
Reviewed-on: http://review.couchbase.org/29768
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>