[xcbf] WSS-XCBF error codes

["Phillip H. Griffin" <phil.griffin@asn-1.com>]

          Monica,
 
In looking again more closely to the WSS-X509 dcoument, I note
 that WSS-XCBF does not mention error codes (section 3.5).
 
Perhaps we should add a section for this. I suggest the following 
mimicing the text in WSS-X509:

  Implementations may use custom error codes defined in private namespaces
  if needed. But it is recommended that they use the error handling codes
defined
  in the WS-Security specification for signature, decryption, encoding and
token
  header errors. When using custom error codes, implementations should be
  careful not to introduce security vulnerabilities that may assist an attacker
in the
  error codes returned .
 
Phil