Make missing ZFS config file failures into warnings, bug #438200

Make missing ZFS config file failures into warnings, bug #438200

Whenever /etc/zfs/{zdev.conf,zpool.cache} was missing, we would fail.
zpool.cache is not generated during cross compilation, which causes a
failure in Catalyst when building LiveCDs. Users that do not follow a
strict set of installation instructions can also suffer failures as a
consequence of this.

zpool.cache includes information about known pools that enables ZFS to
detect situations where an entire pool disappears. It is also required
to do pool import in corner cases, such as those those involving
file-based pools, and has the beneficial effect of reducing pool import
times.  Unconditional omission of zpool.cache in general is not an
option, but for the situations identified, it is okay to convert the
failure into a warning, which is what we do.

Reported-by: Rick Farina <zero_chaos@gentoo.org>
Reported-by: Alexander Zubkov <green@msu.ru>
Signed-off-by: Richard Yao <ryao@gentoo.org>