Catch return errors from zpool commands

Catch return errors from zpool commands

This fixes a bug that can effect first reboot after install
using Dracut.  The Dracut module didn't check the return
value from several calls to z* functions.  This resulted in
"Using no pools available as root" on boot if the ZFS module
didn't auto-import the pools.  It's most likely to happen on
initial restart after a fresh install & requires juggling in
the Dracut emergency holographic shell to fix.

This patch checks return codes & output from zpool list and
related functions and correctly falls into the explicit zpool
import code branch if the module didn't import the pool at load.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>