Should print more helpful error message if 2FA method unsupported

User noticed this because they had 2FA default to SMS method:

Currently, only the "authenticator app" 2FA method is supported (#14). If a user has set a different method as their default, the client will raise an UnexpectedPageError. This isn't very helpful.

I should figure out which URLs the different methods redirect to and let the client detect them to print a more helpful error message, e.g. advising to set the default to the authenticator app.