L2 extension manager don't handle drivers errors

[Slawek Kaplonski]

L2 extension manager which calls L2 extension drivers (like QoS) should handle somehow exceptions from those drivers. For example ext_manager.delete_port(), like in: https://github.com/openstack/neutron/blob/master/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py#L1603 should handle cases where e.g. port already not exists and driver wants to do something with it.
It was discussed in comments to patch https://review.openstack.org/#/c/533318/ and on Neutron CI meeting: http://eavesdrop.openstack.org/meetings/neutron_ci/2018/neutron_ci.2018-01-16-16.01.log.html#l-150