JSON REST API lack of documentation on errors

Is there a reference somewhere to the error codes that the REST API responds with?

I am testing an API client we've built and I'd like to translate the negative integer values into something a little more meaningful, but there seems to be no documentation on errors other than the general HTTP status code 200, 404, 500, etc...

For example, if I create a new address group via API, but then try to create the same group name again, I get the following response:

HTTP/1.1 500 Internal Server Error

...other headers...

BODY contains:

{   "http_method":"POST",   "revision":"c6ccf9fedb18ed33a43ee240a078b94c",   "error":-5,   "status":"error",   "http_status":500,   "vdom":"root",   "path":"firewall",   "name":"addrgrp",   "serial":"FGTXXXXXXXXXXXXX",   "version":"v6.2.1",   "build":932 }

Here, I can only assume that the "-5" for "error" means duplicate entry exists, but there are other possible values we'd like to know what went wrong other than "-2" or something like it.