foo2.cs

        Task AnarcodesWebClientLoop(string[] urls) {
            using (var entryGate = new SemaphoreSlim(10, 10)) {
                var requests = urls.Select(async url => {
                    await entryGate.WaitAsync();

                    var didError = false;
                    string errorMessage = null;
                    try {
                        var req = WebRequest.Create(url);
                        using (var res = (HttpWebResponse)await req.GetResponseAsync())
                        {
                            didError = (int)res.StatusCode < 200 || (int)res.StatusCode >= 300);

                            using (var reader = new StreamReader(res.GetResponseStream()))
                                errorMessage = await reader.ReadToEndAsync();
                        }
                    } catch (Exception e) {
                        didError = true;
                        if (errorMessage == null)
                            errorMessage = e.Message;
                    }

                    // do things with didError and errorMessage here.
                }).ToArray();

                return Task.WhenAll(requests);
            }
        }