Motivation for this change

The download URL will redirect to the login page if credentials are not
present, but will return an error if the credentials are wrong.

Since we know we always expect credentials, we can fail the fetch in
the preHook if they're not provided.

We can't use e.g. builtins.throw because we want the fetchurl derivation
to be evaluated in all cases - because that's the mechanism which allows
you to just manually add the tarball to the Nix store.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Manually tested with:

• nix-build . -A factorio - fails, as expected
• nix-build -E '((import ./. {}).factorio.override { username = ...; token = ...; }' - works, as expected
• nix-build -E '((import ./. {}).factorio.override { username = ...; token = "wrong"; }' - fails, as expected
• nix-build -E '((import ./. {}).factorio.override { username = ...; }' - fails, as expected
• nix-build -E '((import ./. {}).factorio.override { token = ...; }' - fails, as expected