Remote repos in sources.list.d will cause buendia-update to fail when offline

[schuyler]

When apt-get upgrade etc knows about an external apt source (e.g. https://projectbuendia.github.io/) but can't reach the Internet, the process will fail, instead of installing whatever packages it can get to.

This is a likely failure mode currently, since all of our installation methods currently rely on our github.io apt repo.

The Debian installer handles this by having a pre-process that goes through the available apt sources and validates their availability before installing from them. We should probably have some kind of lightweight check that does same.

[zestyping]

This is puzzling to me, because I'm pretty certain that this didn't pose a problem for us in 2015.

In any case, this doesn't seem like a problem that requires a great deal of your current context to diagnose and debug. Thanks for recording it here—I think you can safely move on from this to looking at backup and restore, and I can follow up on this separately.