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

test/mjsunit/harmony/proxies-get-prototype-of.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 target = { target: 1 };
 target.__proto__ = {};
 var handler = { handler: 1 };
 var proxy = new Proxy(target, handler);
@@ skipping 73 matching lines @@
   assertFalse(Object.prototype.isPrototypeOf.call({}, object));
   assertTrue(Object.prototype.isPrototypeOf.call(Object.prototype, object));
 
   handler.getPrototypeOf = function () { return object };
   assertTrue(Object.prototype.isPrototypeOf.call(object, object));
 
   handler.getPrototypeOf = function () { throw "foo" };
   assertThrows(()=> Object.prototype.isPrototypeOf.call(object, object));
   assertThrows(()=> Object.prototype.isPrototypeOf(object));
 })();
+
+
+(function testCrossReamException() {
+  _proto__ = new Proxy({}, {
+  getPrototypeOf() {
+  throw "Should not have been called!"
+  }});
+  var realm = Realm.create();
+  Realm.eval(realm, "Realm.global(0) instanceof Object");
+})();