hideOverlay failures should not cause applet startup failure

On macos and safari, when launching javafx applets, sometimes the applet will fail to launch due to netscape.javascript.JSException when we call Applet.hideOverlay from the FX runtime. This might happen when the overlay on the browser side is not ready yet when we try to call the overlay.

The problem can be reproduced when you try to refresh on a sample applet on javafx.com quickly multiple times.

Anyway, if the overlay is not ready, we should just catch the exception and let the applet continue to launch. This should not prevent applet from launching.