Log at info level the success of a luxi query

So far, any successful query was only logged at debug level, thus leaving
no traces in the logs, unless debugging was enabled. Therefore, we add
a notes about successful completion at info level. Also, repeat a failed
request at WARN level, so that, even if debugging disabled, the request
is available together with the error message in the logs.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>