nixos: throw an error on invalid shell package

All shell packages must export the shellPath passthru

nixos/lib/utils.nix

@@ -12,6 +12,8 @@ rec {
   toShellPath = shell:
     if types.shellPackage.check shell then
       "/run/current-system/sw${shell.shellPath}"
+    else if types.package.check shell then
+      throw "${shell} is not a shell package"
     else
       shell;
 }