Better error Messages when there is no connection to autoupdate

[ostcar]

When there is a problem with the autoupdate, the client shows the connection spinner for ever. (And probably tries forever to reconnect).

Instead, it should show a good error message after some time, that the service is not reachable and the user should try it later. Maybe also show a message in more detail, that the autoupdate service is not reachable, or that it returns a 500er.

This is important for the 4.0, because we will get lots of issues like this one, where it is impossible (or very hard) for the user to know, what is going on.