QtHelp: Check the result of committing collection file transactions

Transaction::commit() ignored the result of QSqlDatabase::commit().
When the commit failed, e.g. because another process held a lock on
the collection file or the disk was full, registering documentation
reported success although nothing was written.

Return the result of the commit and propagate it to the callers.
Keep a failed transaction open, so that it's rolled back on
destruction.

Task-number: QTBUG-150686
Pick-to: 6.12 6.8
Change-Id: I2848066d7b0994d8b1fab4d0271a0355761e78c2
Reviewed-by: Kai Köhne <kai.koehne@qt.io>