Comment 4 for bug 1817953

Reviewed: https://review.openstack.org/639822
Committed: https://git.openstack.org/cgit/openstack/oslo.policy/commit/?id=61732757c0e934a7c61cef2b2d2651f5abbb6fc6
Submitter: Zuul
Branch: master

commit 61732757c0e934a7c61cef2b2d2651f5abbb6fc6
Author: Ben Nemec <email address hidden>
Date: Wed Feb 27 20:53:11 2019 +0000

    Provide more specific error when namespace is missing

    Previously if a non-existent namespace was specified, we just got
    a generic KeyError from stevedore that didn't say a whole lot about
    what went wrong. You pretty much had to go read the code to figure
    out what happened.

    This change adds an explicit check for a missing namespace and raises
    a KeyError with a more specific error message that explains what is
    wrong.

    Change-Id: Ia56d4655d70cee78661567188a977f67b7c3ee78
    Closes-Bug: 1817953