tools: ynl: don't ignore errors in NLMSG_DONE messages

NLMSG_DONE contains an error code, it has to be extracted.
Prior to this change all dumps will end in success,
and in case of failure the result is silently truncated.

Fixes: e4b48ed4

 ("tools: ynl: add a completely generic client")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240420020827.3288615-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>