Send WEBVIEW_ERROR event after CEF finishing loading.

Currently, WEBVIEW_ERROR event is sent to client on CEF::OnLoadError(),
which may cause the error message of wxInfoBar disapear.

CEF::OnLoadEnd will be invoked after CEF::OnloadError, if WEBVIEW_ERROR
event get processed before CEF::LoadEnd, the wxInfoBar's message will
disappear since the webpage will get loaded in CEF::LoadEnd.

CEF will return "data:text/html,chromewebdata" when url is invalid.