AFN抛出error，过去error里返回的数据。

NSData *receiveData =error.userInfo[@"com.alamofire.serialization.response.error.data"];
NSString *receiveStr = [[NSString alloc]initWithData:receiveData encoding:NSUTF8StringEncoding];
// 字符串再生成NSData

NSData * data = [receiveStr dataUsingEncoding:NSUTF8StringEncoding];

// 再解析

NSDictionary *jsonDict = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:nil];
NSLog(@"q请求失败operation----%@",dict );