Change I28323eddcf34fd12002cc706f4a9bcbb3f976ceb introduced
a subtle difference in the types of exceptions that can be
encountered during the operations after creating a subnet.
Before that change, a giant enclosing transaction would provide
REPEATABLE READ guarantees, so once a port was looked up by
IPAM, calling update_port on that port afterwards in the same
transaction would not result in a PortNotFound exception.
The same applied to looking up a router and then updating its
gateway port.
Now that there is no giant enclosing transaction, we have to
capture the possible PortNotFound and RouterNotFound exceptions
that can occur during these post commit subnet create operations.
Closes-Bug: #1644502
Change-Id: I29e2bf7bcda37ebbcee193f94c8b03df90f24ef1
9e006cbe64