fix release note urls, go to archives page from download button if no exe download available

Author: unitycoder

UnityLauncher/Form1.cs

@@ -570,6 +570,7 @@ void DownloadInBrowser(string url)
             else // not found
             {
                 SetStatus("Error> Cannot find installer executable ... opening website instead");
+                url = "https://unity3d.com/get-unity/download/archive";
                 Process.Start(url + "#installer-exe-not-found");
             }
         }

UnityLauncher/Tools.cs

@@ -188,9 +188,10 @@ public static string GetUnityReleaseURL(string version)
                 if (version.Contains("5.6")) padding = "";
                 if (version.Contains("2017.1")) whatsnew = "whatsnew";
                 if (version.Contains("2017.4")) padding = "";
-                if (version.Contains("2018")) padding = "";
+                if (version.Contains("2018.2")) whatsnew = "whatsnew";
+                if (version.Contains("2018.3")) padding = "";
                 if (version.Contains("2019")) padding = "";
-                url = "https://unity3d.com/unity/"+ whatsnew +"/" + padding + version;
+                url = "https://unity3d.com/unity/" + whatsnew + "/" + padding + version;
             }
             else
             if (version.Contains("p")) // patch version