Fixes json parsing and do not throws exception when an error is detected in rawResponse

Fixes json parsing and doesn't throws exceptions in rawResponse when an error is detected, instead of this, it passes the error info to the callback (this is the node convention).

Simple test:

```javascript
kraken.api('Assets', null, function(err, data) {
    if (err) {
        throw err; // error handling left to the user
    }
    console.log(data);
});
```