[QtScript/JavaScriptCore] Make Error prototype inherit Error instance

See http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/892355854c958eec0f8c5ead555fd65a2fc1a6b4

Reason for patch: ECMA-compliance. ECMA-262, 5th edition, 15.11.7.7: "Each NativeError prototype object is an Error object (its [[Class]] is "Error")".

Impact of dropping the patch: Without this patch, the QScriptValue::isError() autotest fails (it calls isError() on each of the builtin Error prototype objects).

Perceived real-world impact of dropping the patch: Little/none. Who relies on this? Code normally handles Error instances, the prototypes are fairly useless (especially since you can't "subclass" Error).

Suggested action: Try to upstream (the patch needs a testcase).