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

test/mjsunit/harmony/regress/regress-2219.js

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 // Flags: --harmony-proxies --expose-gc
 
-var p = new Proxy({}, {getPropertyDescriptor: function() { gc() }});
+var p = new Proxy({}, {getOwnPropertyDescriptor: function() { gc() }});
 var o = Object.create(p);
 assertSame(23, o.x = 23);