Add tests for error traces, and fixes

src/libexpr/eval.hh

@@ -375,9 +375,9 @@ public:
        booleans and lists to a string.  If `copyToStore' is set,
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
+        std::string_view errorCtx,
         bool coerceMore = false, bool copyToStore = true,
-        bool canonicalizePath = true,
-        std::string_view errorCtx = "");
+        bool canonicalizePath = true);
 
     StorePath copyPathToStore(PathSet & context, const Path & path);