Changes to the static hosted `xcop.html`.
Changes to the static hosted `xcop.html`.

* Use `onload` instead of `onreadystatechange` + `readyState === 4`
* Add an `onerror` handler and a new "XCOP-RESPONSE-ERROR" message type.
* Bump the `protocolVersion`.

Note: `onreadystatechange` won't work when you use `onerror`.

YuzuJS/xcop,YuzuJS/xcop