Enhanced ErrorType in Swift. blog; http://blog.sgr-ksmt.org/2016/02/26/enhanced_error_type/ (日本語)

EnhancedErrorType.swift

public protocol EnhancedErrorType: ErrorType, CustomStringConvertible {
    var domain: String { get }
    var code: Int { get }
    var userInfo: [NSObject: AnyObject]? { get }
}

extension EnhancedErrorType {
    var _domain: String {
        return domain
    }
    
    var _code: Int {
        return code
    }

    var domain: String {
        return String(reflecting: self.dynamicType)
    }

    var code: Int {
        return 0
    }

    var userInfo: [NSObject: AnyObject]? {
        return nil
    }
    
    public func toNSError() -> NSError {
        return NSError(domain: domain, code: code, userInfo: userInfo)
    }
    
    var description: String {
        return "\(toNSError())"
    }
}

Usage

enum SomeErrors: EnhancedErrorType {
    case ErrorA
    case ErrorB
    case ErrorC(String)

    var domain: String {
        return "SomeErrorCustomDomain"
    }

    var code: Int {
        switch self {
        case .ErrorA: return -10000
        case .ErrorB: return -10001
        case .ErrorC( _): return -10002
        }
    }

    var userInfo: [NSObject: AnyObject]? {
        switch self {
        case .ErrorA:
            return [NSLocalizedDescriptionKey: "ErrorA occurred."]
        case .ErrorB:
            return [NSLocalizedDescriptionKey: "ErrorB occurred."]
        case .ErrorC(let msg):
            return [NSLocalizedDescriptionKey: "ErrorC occurred. message: \(msg)"]
        }
    }

}

func doSomething() throws {
    throw SomeErrors.ErrorC("wahaha")
}

do {
    try doSomething()
} catch let error {
    print(error as NSError)
    print(error)
    print((error as! SomeErrors).toNSError())
}