Add `toHaveErrorMessage()` matcher

Related to work in #8, we should have a way to handle when an `instance` has an error message we need to match again.

```javascript
const {getByText} = render('listfiles');
expect(getByText('file.txt')).toHaveErrorMessage("Can't find the file");
```

Similar to [jest-dom's API of the same name](https://github.com/testing-library/jest-dom#tohaveerrormessage)