Error message from custom validation not showing

### Environment

- Ember CLI Version: 3.1.4
- Ember CP Validations Version: 4.0.0-beta.2

### Steps to Reproduce

I'm checking for a unique username from an [Airtable](https://airtable.com/) "spreadsheet". The validation seems to be working but the error message is not showing. Not sure why.

```js
const UniqueEmail = BaseValidator.extend({
validate(value, options, model, attribute) {
return base('Base Name').select({
fields: ['email'],
filterByFormula: `{email} = "${value}"`
}).firstPage((err, records) => {
if (err) { throw err; }

if (records.length) {
return 'Email address already exists.';
} else {
return true;
}
});
}
});
```

Any help would be appreciated. Thx!