How to get the JSON error message from HttpRequestException

I have a situation where in I have to extract a Response(HttpResponseMessage) in a catch statement but that I suppose can't be done (using await in catch). Also if i do it after catch , the HttpResponseMessage message gets "Disposed". Code:

 private async void MakeHttpClientPostRequest()
 {
     HttpResponseMessage response = null;
     try
     {
         HttpClient httpClient = new HttpClient();
         httpClient.Timeout = TimeSpan.FromSeconds(15);
         HttpContent httpContent = null;
         if (postJSON != null)
         {
             httpContent = new StringContent(postJSON);
             httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
         }

         response = await httpClient.PostAsync(url, httpContent);
         if (response != null)
         {
             response.EnsureSuccessStatusCode();
             netResults = await response.Content.ReadAsStringAsync();
         }

         if (this.convertedType != null)
         {
             MemoryStream assetReader = GetMemoryStreamFromString(netResults);
             assetReader.Position = 0;
             object value = fromJSON(assetReader, this.convertedType);
             networkReqSuccessWithObjectCallback(this, value);
         }
         else
         {
             //Return netResult as string.
             networkReqSuccessWithStringCallback(this, netResults);
         }
     }

     catch (TaskCanceledException)
     {
         ErrorException ee = null;
         ee = new ErrorException("RequestTimeOut");
         NotifyNetworkDelegates(ee);
     }
     catch (HttpRequestException ex)
     {
         //HERE I have to extract the JSON string send by the server
     }
     catch (Exception)
     {
     }
}

What can be done here ?

Update Previous approach using HttpWebRequest :

public void MakePostWebRequest()
{
    //WebCalls using HttpWebrequest.
    HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
    request.CookieContainer = new CookieContainer();
    request.ContentType = "application/json";
    request.Method = "POST";
    requestState = RequestState.ERequestStarted;
    asyncResult = request.BeginGetRequestStream(new AsyncCallback(GetRequestStream), request);
}


private void GetRequestStream(IAsyncResult asyncResult)
{
    HttpWebRequest request = (HttpWebRequest)asyncResult.AsyncState;
    {
        try
        {
            Stream requestStream = request.EndGetRequestStream(asyncResult);
            if (request != null)
            {
                using (requestStream)
                {
                    StreamWriter writer = new StreamWriter(requestStream);
                    writer.Write(postJSON);
                    writer.Flush();
                }
            }
        }
        catch (WebException we)
        {
        }
    }
}

private void GetResponseStream(IAsyncResult asyncResult)
{
    requestState = RequestState.EResponseStream;

    HttpWebRequest request = asyncResult.AsyncState as HttpWebRequest;
    HttpWebResponse response;
    try
    {
        response = (HttpWebResponse)request.EndGetResponse(asyncResult);
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            netResults = reader.ReadToEnd();
        }
        requestState = RequestState.ERequestCompleted;
    }
    catch (WebException we)
    {
        // failure
        ErrorException ee = null;
        response = we.Response as HttpWebResponse;
        if (response != null)
        {
            using (StreamReader reader = new StreamReader(response.GetResponseStream()))
            {
                //HERE I'm getting the json error message
                netResults = reader.ReadToEnd();
            }
        }
    }
    catch (Exception e)
    {
        networkReqFailedCallback(this, e);
    }
}