Cmd/tailscale/cli: require v4 and v6 default routes to be advertised together

Does this have to be a fatal error? I understand that pretty much everyone would want this still tailscale should be fine with either one as is it is.

The reason it’s fatal as it’s a bad situation for users of an exit node to only have one address family moved: v4 through one place and v6 through their home ISP, or vice versa. Usually it’s a mistake from the person setting up the exit node. That’s why we added the --advertise-exit-node option to be an alias for doing both together.

What’s your use case? Why do you only want to do one or the other?