Emit an error if an error occurs.

When an error occurs, downloadHandler passed an undefined body into `new Buffer` at line 37. This adds a catch for cases where an error is passed to downloadHandler, and emits the error onto the stream.

```javascript
TypeError: First argument needs to be a number, array or string.
    at new Buffer (buffer.js:188:15)
    at Request.downloadHandler [as _callback] (.../gulp-download/index.js:37:57)
```