Confusing output when using --subpath in conjunction with a source that needs to be unpacked

[sternenseemann]

The following prints an error message to stderr, but actually suceeds. It should be clearer that this is just a warning or information about the first try in a series of fallbacks.

$ cabal2nix --supath cabal2nix "https://github.com/nixos/cabal2nix/archive/b3ae6f9240d07ba103f1eb7ab22f6055e9cdb7dc.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  5488  100  5488    0     0  13135      0 --:--:-- --:--:-- --:--:-- 13160
*** parsing cabal file: /nix/store/3kngcq60jypy9nahc0nkxlfdl4rz9pb3-b3ae6f9240d07ba103f1eb7ab22f6055e9cdb7dc.tar.gz/cabal2nix: openBinaryFile: inappropriate type (Not a directory)