public static class DownloadUtilities
{
    public static void DownloadLinks(Dictionary<string, string> files)
    {
        Parallel.ForEach(
            files,
            new ParallelOptions { MaxDegreeOfParallelism = 20 },
            DownloadLink);
    }

    private static void DownloadLink(KeyValuePair<string, string> link, bool retrying = false)
    {
        try
        {
            using (var webClient = new WebClient())
            {
                webClient.Headers.Add("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0)");
                webClient.DownloadFile(new Uri(link.Key), link.Value);
            }
        }
        catch (WebException e)
        {
            if (retrying) { return; } // Silently exit, we're retrying.

            if (e.Status != WebExceptionStatus.ProtocolError)
            {
                throw;
            }

            if (e.Message.Contains("(504) Gateway Timeout") || e.Message.Contains("(403) Forbidden"))
            {
                if (!RetryFailedDownload(link))
                {
                    Program.FailedDownloads.Add(link.Key); // Lets settle for the fact it can't download, and add it to the failed list.
                }
            }
            else
            {
                Logger.Error("Failed to download: " + link.Key);
                Logger.Error(e.Message);
            }
        }
    }

    private static bool RetryFailedDownload(KeyValuePair<string, string> link)
    {
        for (var i = 0; i < 4; i++) // Retry mechanism for 4 trys?
        {
            DownloadLink(link, true);

            if (File.Exists(link.Value)) // It finally managed to download?
            {
                return true;
            }
        }

        return false;
    }
}