Clean error handling on frontend

[PhMemmel]

Whenever the connection is lost or a request (especially the update request which by default is being sent every 10 seconds) cannot be answered by the server, there should be a clean error handling. This also includes proper user feedback, especially whenever a user tries to change the content of the board in any way to warn the user about the fact, that his changes won't be saved.

Maybe the board should be even put into some kind of read only mode as long as the connection to the backend cannot be re-established.