fallback domain when loading webview

[Ivshti]

Summary

When loading the webview, we should have a fallback domain in case the first one fails.

We should code this separately for the windows nad macos versions as they use different webview implementations

Something similar was implemented with the autoupdater: https://github.com/Stremio/stremio-shell/blob/master/autoupdater.js#L10-L14 - which is ALSO something that we need, but we need it for the web URL as well.