fix: incorrect scope in approximateSize func

The following code should give a helpful error about missing a name like this

```
events.js:68
        throw arguments[1]; // Unhandled 'error' event
                       ^
OpenError: #name cannot be `null` or `undefined`
```

Code
```js
var levelup = require('levelup');
levelup('./testdb', function(err, db){
        db.approximateSize(function(err, size){
                if(err)
                        console.log(err);
                console.log(size);
        });
});
```


However we get this instead

```
/Users/sandfox/code/testme/node_modules/levelup/lib/levelup.js:323
      this.emit('error', err)
           ^
TypeError: Object #<Object> has no method 'emit'
    at LevelUP.approximateSize (/Users/sandfox/code/testme/node_modules/levelup/lib/levelup.js:323:12)
    at LevelUP.approximateSize (/Users/sandfox/code/testme/node_modules/levelup/lib/levelup.js:318:12)
    at /Users/sandfox/code/testme/test-error.js:5:5
    at LevelUP.open (/Users/sandfox/code/testme/node_modules/levelup/lib/levelup.js:86:11)
```

I've narrowed it down to scope issue and have hacked up a fix. My javascript skills aren't that great so feel free to bat down or suggest improvements. 
Also I haven't made any proper test cases apart from the above