aptitude shouldn't error about nonexistant pdiffs

[geofft]

Ubuntu doesn't provide pdiffs. apt tries to fetch them. We should make it not print suspicious looking messages when they don't exist because this makes people think auto-update is erroring when it isn't

[broder]

I disagree. They're harmless, they don't cause actual errors, Debian uses pdiffs, and someday Ubuntu might. I think this is a terrible idea from a maintainability standpoint.