SW-816: modified radb insertSpecificationAndTask into insertOrUpdateSpecificationAndTask. This has the advantage that we do not have to reconnect predecessor relations, and in the webscheduler a re-insert of a task will now be visible as an update on an existing task, instead of a delete (disappear) followed by an insert (reappear). Catch task_predecessor_unique PostgresDBQueryExecutionError in the Specification class of the resourceassigner, so scheduling can continue even on tasks which were already known and already had links to predecessors.