Instead of silently doing nothing at all,

Instead of silently doing nothing at all,
warn and return non-zero when the manpath is empty, that is,
when /etc/man.conf is non-existent or unreadable
AND the environment variable MANPATH is unset or empty
AND no directories were given on the command line.

Inspired by the error handling in espie@'s makewhatis(8),
except that one doesn't know about MANPATH.