Display error if no [...nextauth].js found

fixes nextauthjs#647

Author: jaknas

src/server/index.js

@@ -32,6 +32,15 @@ export default async (req, res, userSuppliedOptions) => {
     // safe to return and that no more data will be sent.
     const done = resolve
 
+    if (!req.query.nextauth) {
+      res
+        .status(500)
+        .end(
+          'Error: Cannot find [...nextauth].js in pages/api/auth. Make sure the filename is written correctly.'
+        )
+      return done()
+    }
+
     const { url, query, body } = req
     const {
       nextauth,