Package Manager offline fallback

[Bellisario]

Adding a simple screen message saying "You're offline" would help if someone opens Package Manager and is offline...
Maybe could also be made with simple HTML, as the help page.

Current behavior:

[klange]

The default package manager config does actually have an offline fallback set up, to load a manifest from the CD, but that manifest has been gone for a while. There are four "remotes" currently specified: the CD, a NAT IP address on my local network, the newer DigitalOcean CDN, and the original toaruos.org repository as a fallback.

The CLI does present a message when a manifest fails to load, which regularly prints for the local fallback. I don't want to turn that message into a dialog box for the GUI directly, specifically because of that NAT address case, but it would make sense to catch the case where no repository works and present a message box for that.

[klange]

I added a simple dialog box when no packages are available in da7651c, though I'd like to do something a bit smarter and actually inform about failing repositories, so I'll keep this issue open for now.