breitbandmessung: throw later for unsupported systems

This will provide at least the `name` and `meta` attributes when
evaluating on such a system. `meta.platforms` and therefore
`meta.broken` can then be used to determine if the package can be
evaluated further.

pkgs/applications/networking/breitbandmessung/default.nix

@@ -135,7 +135,7 @@ let
     };
 
     aarch64-darwin = x86_64-darwin;
-  }.${system} or (throw "Unsupported system: ${system}");
+  }.${system} or { src = throw "Unsupported system: ${system}"; };
 in
 stdenv.mkDerivation ({
   pname = "breitbandmessung";