Better unique constraints handling or even updateIfExists/findOneOrCreate

It is too hard handle rows duplication now. As adviced here http://stackoverflow.com/questions/3967226/checking-for-duplicate-keys-with-doctrine-2 we should catch \PDOException with code 23000. This is bad behavior.

So I propose:
a. Add DuplicateRowException wich will throwed by flush method (maybe wrapped PDOException) with method "getDuplicatedProperties" (based on sql error description parsing + mapping)
b. Add method findOneOrCreate (hello, Propel!) or updateIfExists to ObjectInterface, EntityManager
c. Combine a, b