Do not save last sync timestamp in DB if no data has been fetched from server

[pmartin]

If no entry has been fetched from server when syncing (be it because of a network error, because of a wrong configuration, because of a server-side error, ...), we must not save the "last synced timestamp" in the local database.

This way, next time a sync is attempted, data will not be ignored as "more recent than last sync".

[pmartin]

Exceptions are now thrown if something goes wrong while syncing.
And the try/catch block includes the saving of the TS in database.
=> If something goes wrong, the TS will will not be saved in DB, which fixes this issue.