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

test/mjsunit/harmony/regress/regress-crbug-461520.js

 // 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: --harmony-proxies
 
 var fuse = 1;
+
 var handler = {
   get: function() { return function() {} },
-  getPropertyDescriptor: function() {
+  has() { return true },
+  getOwnPropertyDescriptor: function() {
     if (fuse-- == 0) throw "please die";
     return {value: function() {}, configurable: true};
   }
 };
 
 var p = new Proxy({}, handler);
 var o = Object.create(p);
 with (o) { f() }