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

[michaelpj]

i.e. the generated nix files just do hackage.<pkg>.<version> whereas the ones generated from cabal do hackage.<pkg>.<version> or niceError.

[michaelpj]

You might think that this will never happen if you use the hackage.nix/stackage.nix pairs from haskell.nix, since they're updated in sync, so the stackage.nix stuff should never point to something that's too new and so not in the hackage.nix. But: extra-deps exist and will break this.