Return 404 when floating IP pool not found

The FloatingIPPoolNotFound exception is raised only by the
Neutron integration API. When that happens, the exception
handling code should ensure a 404 response code is returned
in order to guarantee consistency with nova-network.

Change-Id: Id8ef7ee2c0d85f8b784b8d1d46c7120331f1e01a
Closes-Bug: 1353936
Merge "Return 404 when floating IP pool not found"