validate return value of ns_cluster:do_change_address

We should not assume that "everything that's not
{address_save_failed,_} is success". Because in this case any change
of do_change_address return values risks causing condition when we
will interpret error as success.

Change-Id: I905def2ae870c2ae190fe2dfdc3b2c9923a41421
Reviewed-on: http://review.couchbase.org/38447
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>