Issue with secure socket creation errors

When using an HTTPS agent, the creation of the secure socket can throw and as a result it raises an exception caught at node process level and the request is hanging forever (or timeout).

https://github.com/delvedor/hpagent/blob/96f45f1d40bfbdfd0fcc84cdba056be6e0fb8f4c/index.js#L102

To reproduce, specify a wrong key/certificate pair to agent options for example :

```js
await request({
method: 'GET',
hostname: SERVER_HOSTNAME,
port: server.address().port,
path: '/',
agent: new HttpsProxyAgent({
keepAlive: true,
keepAliveMsecs: 1000,
maxSockets: 256,
maxFreeSockets: 256,
scheduling: 'lifo',
/**
* Enable SSL 2 ways and generate a 'createConnection' error with wrong key/cert pair
*/
key: 'wrongkey',
cert: 'wrongca',
proxy: `https://${PROXY_HOSTNAME}:${proxy.address().port}`
})
})
```

The PR is on its way if you wish.

Start at index.js around line 102 and reproduce the failure with the wrong key and certificate pair shown in the issue. Trace how secure socket creation errors are handled, then verify that the request reports the error instead of hanging or timing out.

Written by the indexing model from the issue text.