Remove usage print on vdsm-tool.UsageError and use it on ExtraArgsError

This print add usage explanation of vdsm-tool command on error. There is
no need for such print Unless UsageError refers actually to the verb
usage, but currently we use it for any usage error.
On ExtraArgsError catch we add this usage print

Change-Id: I6df376664ffcda66ea19f66e28edc0521566c091
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28406
Reviewed-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>