Deactivate transaction if rollback fails

Deactivate transaction if rollback fails

Fixed regression introduced in 1.2.0b1 due to :ticket:`3934` where the
:class:`.Session` would fail to "deactivate" the transaction, if a
rollback failed (the target issue is when MySQL loses track of a SAVEPOINT).
This would cause a subsequent call to :meth:`.Session.rollback` to raise
an error a second time, rather than completing and bringing the
:class:`.Session` back to ACTIVE.

Fixes: #4050
Change-Id: Id245e8dd3487cb006b2d6631c8bd513b5ce81abe