Notify the end user if the serviceWorker is unavailable

[llaubin]

Hola o/

When sharing a file with someone using Firefox in private mode, the user may be redirected to the home page of the application, with no download available. This is due to navigator.serviceWorker being null, which triggers an exception and so redirects to webroot.

This fix just pops a msgbox to warn the user before being redirected.