Check any sync database operations for improper error management

[mgallien]

Desktop client is relying on a sync database to maintain proper state of a sync folder.
Some access to sync database can generate errors without them being properly handled. Check all such usage to ensure errors are not getting ignored silently.

[mgallien]

@tobiasKaminsky

[allexzander]

• check first that inside SyncJournalDb class we really check any error from low-level methods implementing the DB operations (SqlQuery::exec for example)
• make it impossible to ignore the return values of methods of the SyncJournalDb class