Embarrassing.

That error happens if the map file in use wasn't created at the same time 
as the bootstrap loader, and seems to me to be an extremely good idea.

From my reading of the code, you'll only get that error if you try to boot 
from a old spare disk in conjunction with a /boot containing a LILO map 
which is *not* on that disk, in which case, well, every disk with an MBR 
should have a /boot on it, and then that problem goes away. (Given that 
the alternative is a rather nasty crash without any sort of message, this 
might be a good idea: but perhaps booting should continue and this should 
be demoted to a warning?)

(Also, I'd expect to recover by switching one of your other *currently in 
use* disks over to be the one the MBR is read from. That's the scenario a 
RAIDed boot block is meant to be useful for, not 'grab random disk off 
pile'.)