makeman.lua: Downgrade make showconfig error to warning

The sh-based makeman silently ignored errors from make showconfig.
Ignore errors also from makeman.lua (but emit a warning).

We may want to revisit this in the future, but want makeman.lua to
behave identically for now.

PR: 294822
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56663

(cherry picked from commit ea27ec183d0ff26e1273202841a02041b6d93955)