MB-12141: descriptive error message for Server Groups

Change-Id: I8ab3fc7da3e2ef77cfcb4a9e557e9dcff525bf66
Reviewed-on: http://review.couchbase.org/41359
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>