Simplified demo.html; show proxyError on error

lib/cors-anywhere.js

@@ -336,8 +336,8 @@ exports.createServer = function createServer(options) {
 
   // When the server fails, just show a 404 instead of Internal server error
   server.proxy.on('proxyError', function(err, req, res) {
-    res.writeHead(404, {});
-    res.end();
+    res.writeHead(404, {'Access-Control-Allow-Origin': '*'});
+    res.end('Not found because of proxy error: ' + err);
   });
 
   return server;