EventSource and update fixes

fixes #4005 

- eventsource.php: in case of potential CSRF attack we send an error message from the EventSource to the browser
- eventsource.js: handle undefined data on event
- update.js: in case of error we close the event source - advise the user to reload the page
- update.php: EventSource initialization is now done before we enter the maintenance mode in order to allow browser reload in case of possible CSRF attack

@karlitschek @icewind1991 @MTGap @schiesbn @butonic THX