Log that the check does not understand the extension when verbose
Log that the check does not understand the extension when verbose

Instead of always printing that the check was skipped when in non
verbose mode, log/print it only when verbose mode is enabled and not
otherwise.

Change-Id: I0190b9d82568fb18871591d64ec6bd4695751367

openstack/doc8,doismellburning/doc8,stackforge/doc8,tschuy/doc8