Missing packages and recurrency

After failing to build a package due to unresolved dependencies prevu should spit names and versions of packages that weren't possible to install.

Having this functionality it should be easy to implement a -r switch, which would try to recursively satisfy dependencies prevu'ing them. This way when I'd prevu tomboy, prevu after failing to build tomboy would try to prevu cli-common-dev, etc., doing prevu-updates when needed.