MultiLevelError.swift

import Foundation

public struct AppError: Swift.Error {
    public enum AppErrorReason {
        public enum API: String {
            case wrong_password
            case invalid_email
        }
        public enum Validation: String {
            case invalid_phone_number
            case invalid_top_up_cost
        }
        public enum InternalInconsistency: String {
            case this_shouldnt_happen_report_bug
        }
        case apiError(error: AppErrorReason.API)
        case validationError(error: AppErrorReason.Validation)
        case internalInconsistency(error: AppErrorReason.InternalInconsistency)
    }
    
    let reason: AppError.AppErrorReason
    let userInfo: [String: Any]?
}

extension AppError {
    var reasonCode: String {
        switch reason {
        case .apiError(let error): return error.rawValue
        case .validationError(let error): return error.rawValue
        case .internalInconsistency(let error): return error.rawValue
        }
    }
    var localizedReasonCode: String {
        return NSLocalizedString(reasonCode, comment: "Localized description for error")
    }
}

// Usage examples
func validate(string: String) -> AppError? {
    guard string.characters.count > 8 else {
        return AppError(reason: .validationError(error: .invalid_phone_number), userInfo: nil)
    }
    return nil
}

func apiRequest(with completion: @escaping (Data?, AppError?) -> ()) {
    completion(nil, AppError(reason: .apiError(error: .invalid_email), userInfo: nil))
}

let phone = "+38097"
if let error = validate(string: phone) {
    // process error
    print(error)
}
else {
    // validation successful
}

apiRequest { (data, error) in
    if let error = error {
        // process error
        print(error)
    }
    else {
        // api request successfull
    }
}