[proxy] fixing harmony/proxy.js tests and improving error messages + some drive-by fixes

test/mjsunit/regress/regress-crbug-493568.js

Index: test/mjsunit/regress/regress-crbug-493568.js
diff --git a/test/mjsunit/regress/regress-crbug-493568.js b/test/mjsunit/regress/regress-crbug-493568.js
deleted file mode 100644
index 556230e0ae957aca61d50efac74626efe0341970..0000000000000000000000000000000000000000
--- a/test/mjsunit/regress/regress-crbug-493568.js
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright 2015 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// Flags: --allow-natives-syntax --harmony-proxies
-
-var p = new Proxy({}, { fix: function() { return {}; } });
-
-var obj = {};
-obj.x = p;
-
-Object.preventExtensions(p);