LU-19039 lnetconfig: Fix error string in cyaml output

LU-19039 lnetconfig: Fix error string in cyaml output

String output in yaml only needs to be quoted when beginning with '@',
''', '"', or '- ', or contains ':'.

This corrects the most common error output for `lnetctl ping` errors
to be correct yaml and also cleans up all other error strings output.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I9a8436280b34f82cf78152e488b68c0581cc2a7d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59373
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>