The plaintext version of the server data is only provided for developer convenience. Usually it should be present, or else the reason from the previous error should be present. But if it's not, your app can always get the response body data and attempt to make a string from it, too:
NSData *data = [[ticket objectFetcher] downloadedData];
NSString *str = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
As the Google Data APIs Objective-C Client library is not under active development, this behavior in the library is not likely to change.