Make error log in initialize-sentry-sdk-backend more descriptive

Author: Alex Krawiec

docs/product/sentry-basics/distributed-tracing/initialize-sentry-sdk-backend.mdx

@@ -127,7 +127,7 @@ Make sure to import `instrument.js` at the top of your `server.js` file, as well
     app.use(function onError(err, req, res, next) {
         // The error id is attached to `res.sentry` to be returned
         // and optionally displayed to the user for support.
-        console.log('500?');
+        console.log('500 error thrown!');
         res.statusCode = 500;
         res.end(res.sentry + "\n");
     });