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

test/mjsunit/harmony/regress/regress-405844.js

Index: test/mjsunit/harmony/regress/regress-405844.js
diff --git a/test/mjsunit/harmony/regress/regress-405844.js b/test/mjsunit/harmony/regress/regress-405844.js
deleted file mode 100644
index 4962ec2c78cd8912d454ea34c315a9d10e60a209..0000000000000000000000000000000000000000
--- a/test/mjsunit/harmony/regress/regress-405844.js
+++ /dev/null
@@ -1,13 +0,0 @@
-// Copyright 2014 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: --harmony-proxies --harmony-object-observe
-
-var proxy = new Proxy({}, { fix: function() { return {}; } });
-Object.preventExtensions(proxy);
-Object.observe(proxy, function(){});
-
-var functionProxy = Proxy.createFunction({ fix: function() { return {}; } }, function(){});
-Object.preventExtensions(functionProxy);
-Object.observe(functionProxy, function(){});