dns: make error message match errno

There is a case where an error message coming from dns has `ENOENT`, but the errno is `ENOTFOUND`. This commit, correctly sets the error message to also contain `ENOTFOUND`.

reproduce:
```javascript
require('dns').lookup('does.not.exist', 4, function(err) {
  console.log(err.message, err.errno)
})
```