如果一个函数需要一个返回值，但是在函数内部，我捕获了一个错误并想中断

如果一个函数需要一个返回值，但是在函数内部我遇到了一个错误并想退出该函数，我该如何在C＃中执行此操作...我的问题是（可以随意跳到注释行）：

    public static async Task<string> GetData(string url, string props = "")
    {
        string query = url + "?oauth_token=" + Contract.access_token + props;
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(APIURL + query);
        request.Headers[HttpRequestHeader.IfModifiedSince] = DateTime.Now.ToString();
        request.Method = HttpMethod.Get;

        try
        {
            HttpWebResponse response = (HttpWebResponse)await request.GetResponseAsync();

            Debug.WriteLine(response.ContentType);
            System.IO.Stream responseStream = response.GetResponseStream();
            string data;
            using (var reader = new System.IO.StreamReader(responseStream))
            {
                data = reader.ReadToEnd();
            }
            responseStream.Close();

            APIError error = new APIError ();
            try
            {
                using (MemoryStream stream = new MemoryStream(Encoding.UTF8.GetBytes(data)))
                {
                    DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(APIError ));
                    error = (APIError )serializer.ReadObject(stream);
                }
                MessageBox.Show(error.error_description);
                return null; // I WANT TO BREAK OUT HERE, I really don't want the application to continue its thread... Is there a way of doing this without having to go and code all my controller model calls to try and catch?
            }
            catch (Exception e)
            {
                return data; // The api did not return an error so continue as normal
            }
        }
        catch (Exception ex)
        {
            var we = ex.InnerException as WebException;
            if (we != null)
            {
                var resp = we.Response as HttpWebResponse;
                var code = resp.StatusCode;
                MessageBox.Show("RespCallback Exception raised! Message:{0}" + we.Message);
                Debug.WriteLine("Status:{0}", we.Status);
                return we.Message;
            }
            else
                throw;
        }
    }