Don't error on 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!
Merge pull request #3 from janrain/alorente/no-empty-error

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