I was curious, so I googled.  You may have already found this as well: it's a 'fault code', and apparently there's no standard for such codes.  The choice of 8002 may be modeled on what twisted.web does with exceptions.  Staying consistent with them is not a bad idea if you decide this is a similar enough fault case, but whatever is done it will need to be documented.

On the other hand, I also happened on this document, but I have no idea what its status is in the xmlrpc community:

http://xmlrpc-epi.sourceforge.net/specs/rfc.fault_codes.php