Change NotFound to InstanceNotFound in server_diagnostics.py

According to the get() method in nova/compute/api.py,
the index() method in nova/api/openstack/compute/
contrib/server_diagnostics.py should catch the
InstanceNotFound, rather than the NotFound.
This patch just applies to V2 API, V3 API is already
catching InstanceNotFound.

Change-Id: I9e021837b9050dade9af4725a0712f260eac6f7a
Closes-bug: #1296662
Merge "Change NotFound to InstanceNotFound in server_diagnostics.py"