Make error messages more consistent.
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 <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

ceejatec/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server