Fix exception handling of CLI

Because exc.Unauthorized exception has been removed for commit
"Remove out-dated exceptions"(https://review.openstack.org/#/c/93852/).
but it still in use in ceilometerclient.shell.CeilometerShell#main().
This patch replace exc.Unauthorized with exc.HTTPUnauthorized.
And other three places of exc.NotFound are in use too.

Change-Id: I3e010db20cb6f6636126cceed5c62a9817213739
Closes-bug: #1321649