NSX: sync thread catches wrong exceptions on not found

Previously the sync code expected exc.NoResultFound to be raised if a
port/router/network was not found in the database. This is actually not the
correct exception raised. This error will only occur if an element is deleted
from the db right when the sync thead is run for a specific element.

Change-Id: I3a5c8316ade49004fb9fa0b454435205c543f6ee
Closes-bug: 1309208
(cherry picked from commit 25c79b0ee78639a8abd9ab81c016ed26ba70a5d7)