Add descriptive error message when package version missing

A really minor, minor thing that annoys me is when I forget to specify the package version when
adding a package the following happens:

$ cblrepo add foo
cblrepo: Prelude.tail: empty list

This pull request changes this behavior to:

$ cblrepo add foo
cblrepo: Missing version

I agree that this is just cosmetic, but it's an easy fix so there you go.