disk: zfs: improve error logging for zfs commands

disk: zfs: improve error logging for zfs commands

zfs errors might include a newline in the output (e.g. when trying to
create a mirror on two differently sized disks), which trips up our
task log status parser since that expectes a 'TASK {status}' on the
beginning of the first line.

print the error from zfs into the log and bail out with a short notice
to check the task log

this fixes the 'unknown error' issue in the ui when an error happnes
during the zfs commands

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>