So that when `abstract-leveldown` starts using `level-errors`, and
such a db is wrapped with `levelup`, `levelup` will not rewrap
errors and lose stack trace information in the process. I.e.:

```
// Error created by abstract-leveldown
const err = new ReadError('example')

// Error created by levelup
const wrapped = new ReadError(err)

assert(wrapped === err)
```

Ref Level/community#58