MasterResponse.swift

struct MasterResponse<T: JSONDecodable> {
    let object: T?
    let error: [String: JSON]?
}

extension MasterResponse: JSONDecodable {
    init(json: JSON) throws {
        guard case let .Dictionary(jsonDictionary) = json else {
            let error = JSON.Error.ValueNotConvertible(value: json, to: MasterResponse.self)
            throw error
        }

        let error: [String: JSON]? = {
            guard let errorDictionary = jsonDictionary["error"], case let .Dictionary(error) = errorDictionary else { return nil }
            return error
        }()
        let object: T? = {
            guard let objectDictionary = jsonDictionary["object"], let object = try? T(json: objectDictionary) else { return nil }
            return object
        }()

        guard object != nil || error != nil else { // Enforce having either an object or an error
            let error = JSON.Error.ValueNotConvertible(value: json, to: MasterResponse.self)
            throw error
        }

        self.object = object
        self.error = error
    }
}