zpool should call zfs_nicestrtonum() with non-NULL handle

When zfs_nicestrtonum() is called and there will be an error,
the message is left in libzfs handle, if provided. We can use
this message, to provide better feedback for user.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #12650