Delete explicit raising DBError from transaction

 * We should raise original exception during transaction.
   It allows to see the right response codes from API:
     workbook-create (if already exists) - conflict 409 (was 400)
     action-update (if doesn't exist) - not found 404 (was 400)
     action-delete (if doesn't exist) - not found 404 (was 400)
     and etc.
 * Fixed sqlalchemy test to not assign property 'id' in entities.

Change-Id: I85164c10682da3204884c59a7576593924508647
Merge "Delete explicit raising DBError from transaction"