CBQE-0: add logs if impossible to change autoFailover settings

Change-Id: I305d18109c6b3c358a4232554fdc1a63a37310f7
Reviewed-on: http://review.couchbase.org/26853
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>