MB-8903: Remove all io:format/2

When a connection closed, don't write out an almost meaningless
"vmx: closed", but return {error, closed}.

Change-Id: I06c97d94eabce6f466340f3ba807595caadc618e
Reviewed-on: http://review.couchbase.org/32916
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>