Customizable auth errors

https://firebase.google.com/docs/reference/js/v8/firebase.auth.Error

Allow dev to change error messages by code:

```js
export default {
  'auth/app-deleted': 'App has been deleted',
  'auth/app-not-authorized': 'Current domain is not authorized'
}
```

Maybe allow error messages to be functions to allow for more customization?