return more useful messages when a server function errors

packages/fullstack/src/adapters/axum_adapter.rs

@@ -263,8 +263,8 @@ where
                     let res = service.run(req);
                     match res.await {
                         Ok(res) => Ok::<_, std::convert::Infallible>(res.map(|b| b.into())),
-                        Err(_e) => {
-                            let mut res = Response::new(Body::empty());
+                        Err(e) => {
+                            let mut res = Response::new(Body::from(e.to_string()));
                             *res.status_mut() = StatusCode::INTERNAL_SERVER_ERROR;
                             Ok(res)
                         }