Improve error message when backend is down

[alexhorn]

Instead of "Bei der Verbindung zum Server ist ein Fehler aufgetreten." we should show a different message that suggests that the user is not at fault to improve user experience. (@Kaazyra)

[BuildmodeOne]

As Primuss is currently undergoing maintenance, it would be especially useful to have some information about this as well.

Here we should also provide the user with a somewhat clearer message instead of a technical error message, especially since Primuss maintenance work is communicated quite clearly.