libexpr: Throw the correct error in toJSON

BaseError::addTrace(...) returns a BaseError, but we want to
throw a TypeError instead.

Fixes #6336.

src/libexpr/value-to-json.cc

@@ -84,7 +84,8 @@ void printValueAsJSON(EvalState & state, bool strict,
                 .msg = hintfmt("cannot convert %1% to JSON", showType(v)),
                 .errPos = v.determinePos(pos)
             });
-            throw e.addTrace(pos, hintfmt("message for the trace"));
+            e.addTrace(pos, hintfmt("message for the trace"));
+            throw e;
     }
 }