improve error message and document that the ipv6 prefix is now required

[christian-heusel]

Bug description

When missing the ipv6 section on the just relased v0.23.0-alpha5 a confusing error is printed:

Failed to get headscale configuration error="netip.ParsePrefix(\"\"): no '/'"

Environment

• OS: deployed via docker
• Headscale version: v0.23.0-alpha5

Old config:

ip_prefixes:
  - 100.64.0.0/10

New config:

prefixes:
  # errors without v6 or when v6 is empty
  v6: fd7a:115c:a1e0::/48
  v4: 100.64.0.0/10

• Headscale is behind a (reverse) proxy
• Headscale runs in a container

To Reproduce

Migrate the config ip prefix from the format pre v0.23.0-alpha4 to v0.23.0-alpha5.
With alpha5 it is required that a ipv6 shall be specified and after discussion on discord this was declared a feature. Now that it is required it should be documented and the the error message also could use some improvement so people who face this during migration are not too confused (kradalby said I should open an issue about this small improvement).

[mpldr]

A migration guide would probably be a good section for the docs. After all its not unlikely that there will be at least some additional breaking changes in the future. (Bonus points for a sed command that auto-fixes the config :D)

[x86dev]

Thanks for bringing this bug up, was scratching my head as well what's wrong with my config ...yeah, that indeed should be documented or handled better.

[BJorah]

I also encountered this problem when I use "headscale namespace list"