nsISOAPResponse in Firefox 1.5

 This is an extension of a message which contains extra functions
 such as tracking, where appropriate, the original call that
 produced the response message, identifying the fault, if any,
 and supplying the return value.


 The fault returned in the response, if one was generated. NULL
 if there was no fault.  This does not rely on the response
 parameters having been deserialized.