No way to catch EADDRINUSE

```javascript
const AnyProxy = require("anyproxy");

function startProxy() {
const options = {
port: 8001,
//rule: require("myRuleModule"),
webInterface: {
enable: true,
webPort: 8002
},
throttle: 10000,
forceProxyHttps: false,
wsIntercept: false,
silent: false
};
const proxyServer = new AnyProxy.ProxyServer(options);

proxyServer.on("ready", () => {
console.log("PROXY READY");
});
proxyServer.on("error", e => {
console.log("PROXY ERROR");
/* */
});
proxyServer.start();
}
//starting proxy twice on purpose
startProxy();
startProxy();
```

#### Which platform are you running AnyProxy
Mac

#### The version of the AnyProxy
4.0.12

#### Your expected behavior of AnyProxy
should emit an 'error' if port is in use

#### The actual behavior of AnyProxy
Any proxy emits a 'ready' and later logs that the port is in use. No way to catch error.

#### The log of the error
[AnyProxy Log][2018-11-04 01:33:44]: throttle :10000kb/s
[AnyProxy Log][2018-11-04 01:33:44]: throttle :10000kb/s
[AnyProxy ERROR][2018-11-04 01:33:44]: websocket error, Error: listen EADDRINUSE :::8002
[AnyProxy Log][2018-11-04 01:33:44]: Http proxy started on port 8001
[AnyProxy Log][2018-11-04 01:33:44]: web interface started on port 8002
PROXY READY
[AnyProxy Log][2018-11-04 01:33:44]: Http proxy started on port 8001
[AnyProxy Log][2018-11-04 01:33:44]: web interface started on port 8002
PROXY READY
[AnyProxy ERROR][2018-11-04 01:33:44]: error in websocket proxy: listen EADDRINUSE :::8001,
Error: listen EADDRINUSE :::8001
at Server.setupListenHandle [as _listen2] (net.js:1286:14)
at listenInCluster (net.js:1334:12)
at Server.listen (net.js:1421:7)
at async.series.tipText (/Users/me/Desktop/anyproxy-test/node_modules/anyproxy/proxy.js:205:32)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:607:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:246:17
at iterate (/Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:146:13)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:157:25
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:248:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:612:34
at async.series.tipText (/Users/me/Desktop/anyproxy-test/node_modules/anyproxy/proxy.js:200:11)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:607:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:246:17
at iterate (/Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:146:13)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:157:25
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:248:21
error happened in proxy websocket: { Error: listen EADDRINUSE :::8001
at Server.setupListenHandle [as _listen2] (net.js:1286:14)
at listenInCluster (net.js:1334:12)
at Server.listen (net.js:1421:7)
at async.series.tipText (/Users/me/Desktop/anyproxy-test/node_modules/anyproxy/proxy.js:205:32)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:607:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:246:17
at iterate (/Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:146:13)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:157:25
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:248:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:612:34
at async.series.tipText (/Users/me/Desktop/anyproxy-test/node_modules/anyproxy/proxy.js:200:11)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:607:21
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:246:17
at iterate (/Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:146:13)
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:157:25
at /Users/me/Desktop/anyproxy-test/node_modules/async/lib/async.js:248:21
errno: 'EADDRINUSE',
code: 'EADDRINUSE',
syscall: 'listen',
address: '::',
port: 8001 }

Reproduce the issue by starting two AnyProxy.ProxyServer instances with the example options. Read proxy.js around lines 200-205, where the listen failure is reported, and trace how it reaches the server events. Done means an EADDRINUSE failure is exposed through the documented error-handling path instead of producing a misleading ready event.

Written by the indexing model from the issue text.