Log: schemas.c: treat CIB with unrecognized validate-with better

Test case:
akin to one in commit 35fadfd, take a valid CIB, set validate-with to
"pacemaker-9999.0".

BEFORE:
Your current configuration could only be upgraded to pacemaker-9999.0...
the minimum requirement is pacemaker-2.0.

AFTER:
Your current configuration pacemaker-9999.0 could not validate with any
schema in range [unknown, pacemaker-2.6], cannot upgrade to
pacemaker-2.0.

Note that not special-casing way before update_validation call is
intentional. Now, we can easily decide to allow recovery when whole
CIB is valid at least per a single supported schema and just having
strange valide-with value -- all what'd be needed is to relax (pun...)
requirement on this attribute in the RelaxNG schemas themselves
(but that's just an option, after all).