Catch NeutronClientException when showing a network

When a network id can't be found, neutronclient raise
NetworkNotFoundClient exception, but this exception is not
handled by nova. This will cause a 500 error.
This patch fixes this bug.

Closes-Bug: #1286969
Change-Id: Ia96c9668c74374476d4dccdbdb281e99d91b0088
Merge "Catch NeutronClientException when showing a network"