MB-11710: Expand error message when attempting 1.8.1 join

Tell the user how to fix the failure to join.

Change-Id: Iab2b520ebbb4ebddeacb1903003631adcc2b2e44
Reviewed-on: http://review.couchbase.org/39314
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>