Nice error for missing hackage package versions isn't present if the package lookup comes via a stack snapshot

i.e. the generated nix files just do `hackage..` whereas the ones generated from cabal do `hackage.. or niceError`.

Compare the generated Nix expressions for a package version obtained from a Stack snapshot with those generated from Cabal. Trace the generation entry point for each path and verify that the Stack output includes the same missing-version fallback as the Cabal output. Done means both forms provide the nice error when the Hackage package version is absent.

Written by the indexing model from the issue text.