During upgrade, the config settings aren't always available due to
base.php changes. This fix makes the update info page read the product
name and version from the update template, which already had them.
- 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
