https://stackoverflow.com/a/54564755/1271826

Download.swift

// method with closure that throws

func download(completion: @escaping (URL?) throws -> Void) {
    let url = ...
    URLSession.shared.downloadTask(with: url) { location, _, _ in
        do {
            try completion(location)
        } catch let fileSavingError {
            // Perhaps you're going to do some complicated error handling process that logs the problem or whatever.
            // In this case, I'll just print the error that the `completion` closure threw.
            print(fileSavingError)
        }
    }
}

// caller can then throw errors (notice the `try` references below) that the above method can catch

download { location in
    guard let location = location else { return }
    
    let fileURL = try FileManager.default.url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
    try FileManager.default.copyItem(at: location, to: fileURL)
}