deleting non-existing resource results in internal server error

DELETE request for non-existing resource results in internal server error.
It should return 404 instead.

On neutron side, due to request reorder or out of db sync, neutron odl driver may request delete on non-existing resource.
In that case neutron odl driver should gracefully ignore 404 instead of falling in full sync mode.

https://bugs.launchpad.net/networking-odl/+bug/1489593