Don't error out in the package-already-installed case

This fixes an error message of the form

> ERROR: cabal should only install one package, random-1.0.1.1.
>       However, cabal says it's going to install these packages:
>
>  You may have to remove these packages manually...

This isn't actually an error! As of this patch, you'll see some warnings from cabal and then your build'll proceed normally.