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

test/mjsunit/harmony/proxies-get-prototype-of.js

Index: test/mjsunit/harmony/proxies-get-prototype-of.js
diff --git a/test/mjsunit/harmony/proxies-get-prototype-of.js b/test/mjsunit/harmony/proxies-get-prototype-of.js
index 36f67356d556d39c06253f9e68ce3ee9105ed6d9..557025b478cbca337002aa83079d6c2ad234239e 100644
--- a/test/mjsunit/harmony/proxies-get-prototype-of.js
+++ b/test/mjsunit/harmony/proxies-get-prototype-of.js
@@ -91,3 +91,13 @@ assertSame(Object.getPrototypeOf(proxy3), proxy3_prototype);
   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");
+})();