fix: add error reason to connect

examples/C#/taosdemo/taosdemo.cs

@@ -305,7 +305,7 @@ namespace TDengineDriver
             this.conn = TDengine.Connect(this.host, this.user, this.password, db, this.port);
             if (this.conn == IntPtr.Zero)
             {
-                Console.WriteLine("Connect to TDengine failed");
+                Console.WriteLine("Connect to TDengine failed. Reason: {0}\n", TDengine.Error(0));
                 CleanAndExitProgram(1);
             }
             else