LU-13686 utils: pool_add/remove error code fix

LU-13686 utils: pool_add/remove error code fix

jt_pool_cmd should always return error code, even
if it failed to add/remove just one of OSTs from list.
Before this patch it returned latest command result,
ignoring previous failures.

Change-Id: Ife6cefc006f061b47a1b00daf826d0d1d34fd66c
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/38960
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>