libexpr: add set-to-string error frame

this is much more useful than a frame pointing to the set but claiming
it as a function being called. if the function is actually at fault we
will now point to its attribute, although the position may be slightly
wrong if __toString was defined from set updates or builtin functions.

Change-Id: Ib3eb237a276d94426d9c6fc0e26eea72382d34a2

lix/libexpr/eval.cc

@@ -2309,10 +2309,15 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     auto i = v.attrs->find(ctx.s.toString);
     if (i != v.attrs->end()) {
         Value v1;
-        callFunction(*i->value, v, v1, pos);
-        return coerceToString(pos, v1, context,
-                "while evaluating the result of the `__toString` attribute",
-                coerceMore, copyToStore).toOwned();
+        try {
+            callFunction(*i->value, v, v1, i->pos);
+            return coerceToString(pos, v1, context,
+                    "while evaluating the result of the `__toString` attribute",
+                    coerceMore, copyToStore).toOwned();
+        } catch (EvalError & e) {
+            e.addTrace(ctx.positions[pos], "while converting a set to string");
+            throw;
+        }
     }
 
     return {};