Comment 12 for bug 451366

I've written up a first draft of a solution to this. <https://wiki.ubuntu.com/SoftwarePackageOperations#download-failed> In brief, when a download fails: First check whether there's an Internet connection, and complain about that if necessary. Second, try downloading the package list again, and if that fails, complain that the software channel is not available. But if it succeeds and the URL has changed, try downloading the file again, and if that fails, complain specifically that the file is missing.

Sebastian, does that make sense? The design assumes that the errors will be presented by Aptdaemon, not by USC. That cuts out two layers from the software stack, making it a little more likely that we might be able to get even smarter (e.g. distinguishing between 403 and 404 errors) in future.