libmultipath/checkers: support unsupported paths

libmultipath/checkers: support unsupported paths

We should be able to distinguish the case where a checker
determines a path to be positively down from the case where
the checker fails to obtain necessary information, e.g.
because of a configuration problem (wrong checker).
Use PATH_WILD for the latter case, as it's hardly used now.

Provide a generic message for the situation that a path
checker can't handle a certain path.

Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Martin Wilck <mwilck@suse.com>