Added output to zpool online and offline

I was surprised to discover today that zpool online and
zpool offline don't print any information about why they failed in
many cases, they just return 1 with no information about why.

Let's improve that where we can without changing the library function.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #16244