libexpr: improve error messages for builtins.genericClosure

Show which element(s) are involved at each error point:

- When an element is missing the "key" attribute, show the element
- When an element is not an attribute set, show the element
- When comparing keys fails, show both elements being compared
- When calling operator fails, show which element was being processed

This provides concrete context using ValuePrinter with errorPrintOptions.

Note: errorPrintOptions uses maxDepth=10 by default, which may print
quite deeply nested structures in error messages. This could potentially
be overwhelming, but follows the existing default for error contexts.

tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.err.exp

@@ -0,0 +1,18 @@
+error:
+       … while calling the 'seq' builtin
+         at /pwd/lang/eval-fail-genericClosure-deeply-nested-element.nix:25:1:
+           24| in
+           25| builtins.seq finiteVal (
+             | ^
+           26|   builtins.genericClosure {
+
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-deeply-nested-element.nix:26:3:
+           25| builtins.seq finiteVal (
+           26|   builtins.genericClosure {
+             |   ^
+           27|     startSet = [
+
+       … in genericClosure element { finite = { a0 = { a1 = { a2 = { a3 = { a4 = { a5 = { a6 = { a7 = { a8 = { ... }; }; }; }; }; }; }; }; }; }; «1 attribute elided» }
+
+       error: attribute 'key' missing

tests/functional/lang/eval-fail-genericClosure-deeply-nested-element.nix

@@ -0,0 +1,35 @@
+let
+  finite = {
+    a0 = {
+      a1 = {
+        a2 = {
+          a3 = {
+            a4 = {
+              a5 = {
+                a6 = {
+                  a7 = {
+                    a8 = {
+                      a9 = "deep";
+                    };
+                  };
+                };
+              };
+            };
+          };
+        };
+      };
+    };
+  };
+  finiteVal = builtins.deepSeq finite finite;
+in
+builtins.seq finiteVal (
+  builtins.genericClosure {
+    startSet = [
+      {
+        infinite = import ./infinite-nesting.nix;
+        finite = finiteVal;
+      }
+    ];
+    operator = x: [ (import ./infinite-nesting.nix) ];
+  }
+)

tests/functional/lang/eval-fail-genericClosure-element-missing-key.err.exp

@@ -5,6 +5,6 @@ error:
              | ^
             2|   startSet = [ { nokey = 1; } ];
 
-       … in one of the attrsets generated by (or initially passed to) builtins.genericClosure
+       … in genericClosure element { nokey = 1; }
 
        error: attribute 'key' missing

tests/functional/lang/eval-fail-genericClosure-element-not-attrset.err.exp

@@ -5,6 +5,6 @@ error:
              | ^
             2|   startSet = [ "not an attrset" ];
 
-       … while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure
+       … in genericClosure element "not an attrset"
 
        error: expected a set but found a string: "not an attrset"

tests/functional/lang/eval-fail-genericClosure-keys-incompatible-types.err.exp

@@ -5,6 +5,8 @@ error:
              | ^
             2|   startSet = [
 
-       … while comparing the `key` attributes of two genericClosure elements
+       … while comparing element { key = "string"; }
 
-       error: cannot compare a string with an integer
+       … with element { key = 1; }
+
+       error: cannot compare a string with an integer; values are "string" and 1

tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.err.exp

@@ -5,6 +5,8 @@ error:
              | ^
             2|   startSet = [
 
-       … while comparing the `key` attributes of two genericClosure elements
+       … while comparing element { key = { }; }
 
-       error: cannot compare a set with a set; values of that type are incomparable
+       … with element { key = { }; }
+
+       error: cannot compare a set with a set; values of that type are incomparable (values are { } and { })

tests/functional/lang/eval-fail-genericClosure-operator-not-list.err.exp

@@ -5,6 +5,8 @@ error:
              | ^
             2|   startSet = [ { key = 1; } ];
 
+       … while calling operator on genericClosure element { key = 1; }
+
        … while evaluating the return value of the `operator` passed to builtins.genericClosure
 
        error: expected a list but found a string: "not a list"

tests/functional/lang/infinite-nesting.nix

@@ -0,0 +1,4 @@
+let
+  mkInfinite = i: { "a${toString i}" = mkInfinite (i + 1); };
+in
+mkInfinite 0