[FIX] http dispatch: make sure transaction rollback happens in all cases when request...

[FIX] http dispatch: make sure transaction rollback happens in all cases when request handling raised (to be refactored)

Not doing so leads to cases where a transaction is committed
even after failing to validate a Python constraint.
This patch attempts to keep the rollback() logic next to the
corresponding commit(), and makes the rollback() more
explicit even if close() without commit() would also rollback
implicitly.

bzr revid: odo@openerp.com-20140317163228-zqbce8f0rgcg0pt4