Invalid error response fixes

Author: shefin

Source/Podio .NET/Podio.cs

@@ -195,7 +195,7 @@ internal async Task<T> Get<T>(string url, Dictionary<string, string> requestData
                 {
                     throw new PodioInvalidJsonException((int)response.StatusCode, new PodioError
                     {
-                        Error = "Invalid json string",
+                        Error = "Error response is not a valid Json string.",
                         ErrorDescription = ex.ToString(),
                         ErrorDetail = responseBody
                     });