Also forward error requests to the proxy

Fixes webpack#2380

lib/Server.js

@@ -288,7 +288,7 @@ class Server {
         this.websocketProxies.push(proxyMiddleware);
       }
 
-      this.app.use((req, res, next) => {
+      const handle = (req, res, next) => {
         if (typeof proxyConfigOrCallback === 'function') {
           const newProxyConfig = proxyConfigOrCallback();
 
@@ -319,7 +319,11 @@ class Server {
         } else {
           next();
         }
-      });
+      };
+
+      this.app.use(handle);
+      // Also forward error requests to the proxy so it can handle them.
+      this.app.use((error, req, res, next) => handle(req, res, next));
     });
   }
 

test/server/proxy-option.test.js

@@ -326,6 +326,19 @@ describe('proxy option', () => {
       // Parse application/json
       proxy.use(bodyParser.json());
 
+      // This forces Express to try to decode URLs, which is needed for the test
+      // associated with the middleware below.
+      proxy.all('*', (_req, res, next) => {
+        next();
+      });
+      // We must define all 4 params in order for this to be detected as an
+      // error handling middleware.
+      // eslint-disable-next-line no-unused-vars
+      proxy.use((error, proxyReq, res, next) => {
+        res.status(500);
+        res.send('error from proxy');
+      });
+
       proxy.get('/get', (proxyReq, res) => {
         res.send('GET method from proxy');
       });
@@ -372,6 +385,10 @@ describe('proxy option', () => {
       });
     });
 
+    it('errors', (done) => {
+      req.get('/%').expect(500, 'error from proxy', done);
+    });
+
     it('GET method', (done) => {
       req.get('/get').expect(200, 'GET method from proxy', done);
     });