LU-17431 utils: fix various ret codes in lctl

LU-17431 utils: fix various ret codes in lctl

When nodemap_cmd() returns an error, use errno to print
correct return code.
Make get_mgs_device() return an errno in case of failure.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I74f6e27fc17158bf454f0d8be490a087aa137079
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54501
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>