NSError received in equestFailed:withError: has userInfo always null

[SorinAntohi]

Hello,
i'm using your code to pull and push data to Twitter from my app. the problem is that when i'm receiving the NSError for a failed request i receive for example just:

Error Domain=HTTP Code=403 "The operation couldn’t be completed. (HTTP error 403.)"
for a double tweet post i should have received also "Reason: Status is a duplicate" but the userInfo proprety of the NSError is null.

How can i decide when is a double post in order to show the user a custom error message?