Bad error when git clone fails

[nh2]

In #129, when nix-prefetch-git fails because of a too old version of nix being used, this is printed (after a successful clone):

error: current Nix store schema is version 10, but I only support 7

Nevertheless, when that happens stack2nix lets it continue, trying to clone my git URL as hg instead, which obviously fails and then gives a bad error message.

[domenkozar]

That's an issue in NixOS/cabal2nix#260 (comment)