sphinxext: Warn if namespace or command pattern invalid
sphinxext: Warn if namespace or command pattern invalid

Calling the Sphinx extension with an invalid namespace or command
pattern currently results in a silent failure. Correct this oversight by
raising a warning, which will cause a build failure is
'warning-is-error' is enabled.

Change-Id: I63d18c86931d752c3a680e84a69df53bc04e7dfd
Signed-off-by: Stephen Finucane <492121341a95b3c3aab646bed44634f739dd019b@redhat.com>

dtroyer/cliff,dtroyer/cliff,openstack/cliff,openstack/cliff