toJSON: report error position for fancier output

Given flake:

```nix
{ description = "nix json error provenance";
  inputs = {};
  outputs = { self }: {
    jsonFunction = _: "function";
    json = builtins.toJSON (_: "function");
  };
}

```
- Before:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON
```

- After:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON

       at /nix/store/b7imf1c2j4jnkg3ys7fsfbj02s5j0i4f-source/testflake/flake.nix:4:5:

            3|   outputs = { self }: {
            4|     jsonFunction = _: "function";
             |     ^
            5|     json = builtins.toJSON (_: "function");
```

src/libexpr/primops.cc

@@ -1008,7 +1008,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                     if (i->name == state.sStructuredAttrs) continue;
 
                     auto placeholder(jsonObject->placeholder(key));
-                    printValueAsJSON(state, true, *i->value, placeholder, context);
+                    printValueAsJSON(state, true, *i->value, pos, placeholder, context);
 
                     if (i->name == state.sBuilder)
                         drv.builder = state.forceString(*i->value, context, posDrvName);
@@ -1687,7 +1687,7 @@ static void prim_toJSON(EvalState & state, const Pos & pos, Value * * args, Valu
 {
     std::ostringstream out;
     PathSet context;
-    printValueAsJSON(state, true, *args[0], out, context);
+    printValueAsJSON(state, true, *args[0], pos, out, context);
     mkString(v, out.str(), context);
 }