handle HttpClient error

I'm trying to intercept the HTTP client error in case the server is non-existent or down.

i use this code:

HttpResponseMessage response;
            try
            {
                 response = await _client.GetAsync("CallApiXamarin/" + User);
            }
            catch (Exception ex) when (ex is TaskCanceledException || ex is OperationCanceledException)
            {

                throw;
            }

But the APP hangs in debugging by showing this message:

Unhandled Exception:

System.Threading.Tasks.TaskCanceledException: A task was canceled.

What should I do?