queryport: Detect and return error for stats response

Statistics response message needs to be parsed and checked
whether error field is set. If an error message is
set in the stats response message, error needs to be
returned by client.Statistics().

Change-Id: I9da6ff2ea6626a9b661fd0a60c40b55da3ea888a
Reviewed-on: http://review.couchbase.org/43326
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>