Close the redis connection on error

Before losing the reference to `this.client`, forcibly close the
connection to the Redis server.

If the Redis server couldn’t be reached in the first place
(`ECONNREFUSED` on connection), this also prevents node-redis from
auto-reconnecting indefinitely.

---

I couldn't come up with a simple test case, but here is a meaningful use case:

When the Redis server isn't available, `client.stop()` doesn't work as expected, as something keeps the event loop busy:

```javascript
var Catbox = require('./');

var client = new Catbox.Client({
	engine: 'redis',
	partition: 'examples'
});

client.start(function (err) {
	console.log(err);
	client.stop();
});
```

outputs:

```
~/tmp ❯❯❯ node test.js
[Error: Redis connection to 127.0.0.1:6379 failed - connect ECONNREFUSED]

```
(the program hangs there forever and needs to be stopped using `^C`)

Whereas if the Redis server is available:

```
~/tmp ❯❯❯ node test.js
undefined
~/tmp ❯❯❯ 
```