json: Modify API to use return codes

Modify the auditing JSON API to return a response code, as the consensus
was that the existing error handling was aesthetically displeasing.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
