Swift Error handling, bridging the gap between Error and NSError.

ACError.swift

import Foundation

public typealias ACError = NSError

extension ACError {
    public convenience init(message: String) {
        var infoDict = [String: Any]()
        infoDict[NSLocalizedDescriptionKey] = message
        self.init(domain: "com.appcron", code: 0, userInfo: infoDict)
    }

    public var message: String {
        // Use localizedDescription instead in infoDict,
        // since some CoreData errors have a localizedDescription but no entry in the dictionary.
        localizedDescription
    }
}

extension Error {
    public var message: String {
        if let errorWithMessage = self as? ErrorWithMessage {
            errorWithMessage.message
        } else {
            // Error can always be cast to NSError
            (self as NSError).message
        }
    }
}

// This protocol enables other structs of Type Error to override the message property.
public protocol ErrorWithMessage {
    var message: String { get }
}