Switch to offline mode when server is in maintenance mode

[SystemKeeper]

While we react correctly to a 503 http error, we fail to take that into account when retrieving messages.

How to test:

• Enter a conversation while maintenance mode is off
• Enable maintenance mode on the server
• Wait until the timeout for retrieving new messages ends

Without this PR: We end up in an infinity loop and try to request new messages multiple times a second

With this PR: We switch to offline mode, show the maintenance message and don't try to retrieve messages anymore.