Error for IFD usage should be better

[infinisil]

Issue description

For context, IFD is disallowed for packages in Nixpkgs.

When you try to declare a package using IFD, the pkgs/by-name check gives a poor error message:

error: path '/nix/store/2vvz5866f1ljc547rfbqj8xvsr2pxg6p-source.drv' is not valid

See NixOS/nixpkgs#261052

This should be improved, though that might be better done in Nix itself.