There are a lot of tty_warn(0,...) and syswarn(0,...) which probably ought
to be tty_warn/syswarn(1,...) to force an error exit. However some are
used in interactive parts (eg opening a continuation archive) where there
is a separate retry loop.
So we just pass a failure code out to main() - how quaint!
This should now cause the NetBSD build to fail when gzip tries to write
to a non-existant directory.
(I suspect there are still many errors that don't get reported correctly.)