Better error messages on "known unsupported system"?

[hasufell]

e.g. #1078

There are two ways to do such warnings:

• the ghcup bootstrap script
• ghcup binary when doing bindist selection

For the latter we could emit a warning IFF:

• the platform/distro selected is not "unknown linux" (otherwise the warning will show on all non-major distros)
• we have proper version information (e.g. FreeBSD 13.0)
• there is no explicit match for this version, except the fallback "unknown_version"

This would require to carry additional information about our bindist matching (is it a proper hit or the catch-all case).

However, this would also require to review the metadata file, because it is well possible we use "unknown_version" rather eagerly, e.g. because the bindists built on that platform are very portable anyway.