Login screen: warn when not connected

[mahibi]

1. disable phones internet connection
2. try to login to a nextcloud instance

Expected

• Already warn on the screen itself that internet connection fails (that might be the only place where this check makes sense, in the other places the connection to the instance should be checked, see talk-android doesn't work without access to google.com #6240)

Currently

See "Talk app is not installed on the server you tried to authenticate against" and try to find out for a too long time whats going wrong 🙈