feat: allow sending a custom proxy timeout error

When using `proxyTimeout` it's very difficult to tell the difference
between a regular socket hangup and a timeout because, in both cases, an
`ECONNRESET` error is thrown. Ideally we should be able to identify when
a proxy request has failed because it took too long, this would allow us
to do things like send appropriate `504` status code.

Suddenly throwing a different error would probably be considered a
breaking change because it's possible that users of http-proxy are
relying on the `ECONNRESET` error. I decided to add the custom timeout
error behind a new option for now so that people can opt into using it.

If you set this option:

```js
var proxy = httpProxy.createProxyServer({
  target: 'http://example.com',
  proxyTimeout: 100,
  proxyTimeoutCustomError: true
});
```

Then the error that gets thrown will have a message of `"The proxy
request timed out"` and a code of `ETIMEDOUT` to match Node.js:
https://nodejs.org/api/errors.html#common-system-errors

This allows for custom error handling code like this:

```js
proxy.on('error', function(err, req, res) {
  if (err.code === 'ETIMEDOUT') {
    res.writeHead(504);
  } else {
    res.writeHead(503);
  }
  // ...
});
```

Resolves http-party#1331.

Author: rowanmanning

README.md

@@ -374,6 +374,7 @@ proxyServer.listen(8015);
      ```
 *  **headers**: object with extra headers to be added to target requests.
 *  **proxyTimeout**: timeout (in millis) for outgoing proxy requests
+*  **proxyTimeoutCustomError**: true/false, default: false - specify whether you want to throw a custom `ETIMEDOUT` error when the `proxyTimeout` is reached. If false then the default `ECONNRESET` error will be thrown.
 *  **timeout**: timeout (in millis) for incoming requests
 *  **followRedirects**: true/false, Default: false - specify whether you want to follow redirects
 *  **selfHandleResponse** true/false, if set to true, none of the webOutgoing passes are called and it's your responsibility to appropriately return the response by listening and acting on the `proxyRes` event

lib/http-proxy/passes/web-incoming.js

@@ -138,7 +138,12 @@ module.exports = {
     // show an error page at the initial request
     if(options.proxyTimeout) {
       proxyReq.setTimeout(options.proxyTimeout, function() {
-         proxyReq.abort();
+        if (options.proxyTimeoutCustomError) {
+          var timeoutError = new Error('The proxy request timed out');
+          timeoutError.code = 'ETIMEDOUT';
+          return proxyReq.destroy(timeoutError);
+        }
+        proxyReq.destroy();
       });
     }
 

test/lib-http-proxy-passes-web-incoming-test.js

@@ -287,6 +287,41 @@ describe('#createProxyServer.web() using own http server', function () {
     }, function() {}).end();
   });
 
+  it('should proxy the request with custom timeout errors (proxyTimeoutCustomError)', function(done) {
+    var proxy = httpProxy.createProxyServer({
+      target: 'http://127.0.0.1:45002',
+      proxyTimeout: 100,
+      proxyTimeoutCustomError: true
+    });
+
+    require('net').createServer().listen(45002);
+
+    var proxyServer = http.createServer(requestHandler);
+
+    var started = new Date().getTime();
+    function requestHandler(req, res) {
+      proxy.once('error', function (err, errReq, errRes) {
+        proxyServer.close();
+        expect(err).to.be.an(Error);
+        expect(errReq).to.be.equal(req);
+        expect(errRes).to.be.equal(res);
+        expect(new Date().getTime() - started).to.be.greaterThan(99);
+        expect(err.code).to.be('ETIMEDOUT');
+        done();
+      });
+
+      proxy.web(req, res);
+    }
+
+    proxyServer.listen('8087');
+
+    http.request({
+      hostname: '127.0.0.1',
+      port: '8087',
+      method: 'GET',
+    }, function() {}).end();
+  });
+
   it('should proxy the request and handle timeout error', function(done) {
     var proxy = httpProxy.createProxyServer({
       target: 'http://127.0.0.1:45001',