Silence cp error when optional ZFS files are unavailable

Users were seeing the following error message printed:

cp: cannot stat ‘/etc/zfs/zdev.conf’: No such file or directory

Both zdev.conf and zpool.cache are optional files. We print a warning
when they are absent, but cp printed its own error in addition to our
warning. We suppress that.

Signed-off-by: Richard Yao <ryao@gentoo.org>