Make error messages more consistent.

Smae error message is used for the identical errors returned from
parse_failover_args and from graceful failover routine itself.

Change-Id: I0e0c84e6ea906512b65911a2b6e7b2193c8de95f
Reviewed-on: http://review.couchbase.org/38905
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>