Transaction.begin() didn't do anything.

begin() is supposed to abort the current transaction, but
Transaction.begin() did not.  Calling begin() on a transaction
*manager* worked fine, and is the intended way to do a begin()
in 3.3.  But calling begin() on a Transaction object is still
very easy to do (e.g., the older get_transaction().begin()
spelling still works), and shouldn't be a subtle disaster.