neutronv2: translate 401 and 404 neutron client errors in show_port

This translates the PortNotFoundClient and Unauthorized exceptions from
the neutron client's show_port call into the corresponding nova
exceptions. This is useful in the case of a compute API calling
network_api.show_port, e.g. the os-interface API.  The compute API
should expect a NovaException back from the network API rather than a
NeutronClientException (since the compute API shouldn't know if it's
talking to nova-network or neutron).

Long-term we should refactor more of the neutronv2 API code to re-use
the show_port method since there are several places where
neutron.show_port is called directly with additional filtering, etc,
but they potentially also have the same exception translation issues.

Partial-Bug: #1378389

Change-Id: Ic76d3af6b31e501d9ded5f60da9106d5ef88f4b7
Merge "neutronv2: translate 401 and 404 neutron client errors in show_port"