Handle invalid warehouse/account errors correctly

[sgrebnov]

When warehouse or account identifier are invalid there is no authn_method field returned and the lib is unable to parse response as a result error message is very uninformative and hard to troubleshoot ('unable to parse response'.)

Example response

{
  "data" : {
  "internalError" : false,
  "errorCode" : "390201",
  "age" : 0
},
  "code" : "390201",
  "message" : "The requested warehouse does not exist or not authorized.",
  "success" : false,
  "headers" : null
}

As AuthErrorResponseData is the last one in AuthResponse enum, it will be parsed last so it is safe to make the fields optional (tested).

[andrusha]

Thanks, that ties back to a number of issues, including #38 and #31