Return HTTP 401 when token is missing

[jameswestman]

When the repository returns an HTTP 401 error, flatpak is supposed to request a token (if it hasn't already) using the authenticator. However, we currently return a 403, which causes the transaction to fail instead.

I'm marking this as a draft because I haven't been able to fully test it, due to https://github.com/flatpak/flatpak/issues/4862.