MB-7204: Invalid checkpoint displays error from cbstats

Change-Id: Ic429130e89d3014c22e28c6501647d06da027ab4
Reviewed-on: http://review.couchbase.org/24146
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>