improve test debugging and loading of cached redirects json file (#23584)

* improve test debugging and loading of cached redirects json file

* exception for testing 500 page itself

middleware/handle-errors.js

@@ -27,6 +27,16 @@ async function logException(error, req) {
 }
 
 export default async function handleError(error, req, res, next) {
+  // When you run tests that use things doing get() requests in
+  // our supertest handler, if something goes wrong anywhere in the app
+  // and its middlewares, you get a 500 but the error is never displayed
+  // anywhere. So this is why we log it additionally.
+  // Note, not using console.error() because it's arguably handled.
+  // Some tests might actually expect a 500 error.
+  if (process.env.NODE_ENV === 'test') {
+    console.warn('An error occurrred in some middleware handler', error)
+  }
+
   try {
     // If the headers have already been sent or the request was aborted...
     if (res.headersSent || req.aborted) {