Retry http error related to DNS resolution failure. (#4110)

Author: TingluoHuang

src/Sdk/Common/Common/VssHttpRetryMessageHandler.cs

@@ -1,14 +1,14 @@
-using GitHub.Services.Common.Diagnostics;
-using System;
+using System;
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics;
+using System.Linq;
 using System.Net;
 using System.Net.Http;
 using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
-using System.Collections.Generic;
-using System.Linq;
+using GitHub.Services.Common.Diagnostics;
 using GitHub.Services.Common.Internal;
 
 namespace GitHub.Services.Common

src/Sdk/Common/Common/VssNetworkHelper.cs

@@ -146,6 +146,7 @@ private static bool IsTransientNetworkExceptionHelper(
                     sockEx.SocketErrorCode == SocketError.TimedOut ||
                     sockEx.SocketErrorCode == SocketError.HostDown ||
                     sockEx.SocketErrorCode == SocketError.HostUnreachable ||
+                    sockEx.SocketErrorCode == SocketError.HostNotFound ||
                     sockEx.SocketErrorCode == SocketError.TryAgain)
                 {
                     return true;