Another fix for leaking error objects. User code can overwrite

Another fix for leaking error objects. User code can overwrite
ReferenceError.prototype.__proto__ which will make "error instanceof
Error" fail. However, the ReferenceError.prototype object itself
cannot be modified. Therefore, the error checks must check for
concrete error instances instead of only checking for Error.

Committed: http://code.google.com/p/v8/source/detail?r=6450

[Søren Thygesen Gjesse, 2011-01-25 08:48:13]

LGTM