this is a good reason for nintendo to support downloadable updates (from a computer), and then update via physical media, like an SD card..
as for the problem, if it's erroring out, and still after a day or so with no luck, i'd have to question if it's something about your connection. even if the prelim test goes ok.. i wonder if there isn't something blocking the connection for file transfers. your wifi doesn't have any firewalls or proxies or anything does it? there's always the suggestion to consider trying from a different connection/location (another house, office, etc.).