AlamofireObjectMapper Demo from http://stackoverflow.com/a/40792947/1271826

AlamofireObjectMapperDemo.swift

//
//  ViewController.swift
//  MyApp4
//
//  Created by Robert Ryan on 3/10/17.
//  Copyright © 2017 Robert Ryan. All rights reserved.
//

import UIKit
import Alamofire
import AlamofireObjectMapper
import ObjectMapper

class CurrencyFailureResponse: Mappable {
    var success: Bool?
    var message: String?
    
    required init?(map: Map) { }
    
    func mapping(map: Map) {
        success    <- map["success"]
        message    <- map["message"]
    }
}

extension CurrencyFailureResponse: CustomStringConvertible {
    var description: String { return "<CurrencyFailureResponse; success = \(success); message = \(message)>" }
}

class ModelCurrency: Mappable {
    
    var success   : Bool?
    var terms     : String?
    var privacy   : String?
    var timestamp : CGFloat?
    var source    : String?
    var quotes    : [Quote]?
    
    required init?(map: Map) {
        guard let success = map.JSON["success"] as? Bool, success else { return nil }
    }
    
    func mapping(map: Map) {
        
        success    <- map["success"]
        terms      <- map["terms"]
        privacy    <- map["privacy"]
        timestamp  <- map["timestamp"]
        source     <- map["source"]
        
        var dictionary: [String: CGFloat]?
        dictionary <- map["quotes"]
        quotes = dictionary?.map { return Quote(name: $0.key, val: $0.value) }
    }
}

extension ModelCurrency: CustomStringConvertible {
    var description: String { return "<Model Currency; success = \(success); terms = \(terms); privacy = \(privacy); timestamp = \(timestamp); source = \(source); quotes = \(quotes)>" }
}

class Quote {
    
    var name : String?
    var val : CGFloat?
    
    init(name: String?, val: CGFloat?) {
        self.name = name
        self.val  = val
    }
    
}

extension Quote: CustomStringConvertible {
    var description: String { return "<Quote; name = \(name); val = \(val)>" }
}


class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let url = "http://..."
        let parameters = ["foo": "baz"]
        Alamofire.request(url, method: .post, parameters: parameters, encoding: URLEncoding.default)
            .responseObject { (response: DataResponse<ModelCurrency>) in
                switch response.result {
                case .success(let value):
                    print("\(value)")
                case .failure(let error):
                    print("\(error)")
                }
            }
            .responseObject { (response: DataResponse<CurrencyFailureResponse>) in
                switch response.result {
                case .success(let value):
                    print("\(value)")
                case .failure(let error):
                    print("\(error)")
                }
        }
    }
    
    
}

In the above snippet, I'm assuming that if successful, it returns JSON like shown here, but upon failure, you get some JSON like:

{"success":false, "message": "some error message here"}

You can (a) make the model object fail mapping if success was not true; and (b) set up another wrapper to handle the error.