add support for netstandard 2.0

Author: pfriesch

src/LlmTornado/EndpointBase.cs

@@ -400,7 +400,11 @@ private async Task<RestDataOrException<HttpResponseMessage>> HttpRequestRawWithA
             }
 
             ErrorHttpCallResult error = new ErrorHttpCallResult(result.StatusCode, resultAsString, null);
+#if MODERN
             HttpRequestException e = new HttpRequestException(httpRequestError: HttpRequestError.Unknown, message: resultAsString, statusCode: result.StatusCode);
+#else
+            HttpRequestException e = new HttpRequestException($"{resultAsString} (status code: {result.StatusCode})");
+#endif
             result.Dispose();
 
             if (Api.HttpStrict)