Improve error reporting when cabal2nix is called with a syntactically bogus package id.
Improve error reporting when cabal2nix is called with a syntactically bogus package id.

    $ cabal2nix cabal://ses-html.cabal
    cabal2nix.hs: invalid Haskell package id "ses-html.cabal"

This patch fixes https://github.com/NixOS/cabal2nix/issues/172.

bennofs/cabal2nix,psibi/cabal2nix,jb55/cabal2nix,Fuuzetsu/cabal2nix,dmalikov/cabal2nix,gridaphobe/cabal2nix,spencerjanssen/cabal2nix