Extend the syncronisation state

Extend the syncronisation state from date and time to date and time and state (failure, success) on all plattforms

We can assume the specified synchronization date is the last successful sync.

this is not true with a self hosted installation. i can shutdown the server, sync on my mobile and the mobile says failure and the date is refreshed at the time of the failure without status information

the problem is still not fixed. i shutdown my onprem instance and the sync date and time updated, very bad