Friendlier exceptions for Auth testing

This change also lets you write the auth code in the
app.config file without the need to recompile/debug.

TLSharp.Core/TelegramClient.cs

@@ -294,4 +294,9 @@ namespace TLSharp.Core
         {
         }
     }
+
+    public class InvalidPhoneCodeException : Exception
+    {
+        internal InvalidPhoneCodeException(string msg) : base(msg) { }
+    }
 }