nix flake show: log attribute name that "must be a derivation"

I would run `nix flake show` on a flake than hit:

===
        ├───ihaskell: package 'ihaskell-wrapper'
        ├───ihaskell-96: package 'ihaskell-wrapper'
        ├───ihaskell-96-dev: package 'ghc-shell-for-ihaskell-0.10.4.0'
error: expected a derivation
===
and it is not obvious what package is the culprit here since nix stops
rightaway.


Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/nix/flake.cc

@@ -1303,8 +1303,10 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                         try {
                             if (visitor.isDerivation())
                                 showDerivation();
-                            else
+                            else {
-                                throw Error("expected a derivation");
+                                auto name = visitor.getAttrPathStr(state->s.name);
+                                logger->warn(fmt("%s is not a derivation", name));
+                            }
                         } catch (IFDError & e) {
                             if (!json) {
                                 logger->cout(

tests/functional/flakes/show.sh

@@ -107,3 +107,26 @@ in
 assert show_output.packages.${builtins.currentSystem}.default == { };
 true
 '
+
+
+# Test that nix keeps going even when packages.$SYSTEM contains not derivations
+cat >flake.nix <<EOF
+{
+  outputs = inputs: {
+    packages.$system = {
+      drv1 = import ./simple.nix;
+      not-a-derivation = 42;
+      drv2 = import ./simple.nix;
+    };
+  };
+}
+EOF
+nix flake show --json --all-systems > show-output.json
+# shellcheck disable=SC2016
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.packages.${builtins.currentSystem}.not-a-derivation == {};
+true
+'
+