Failed Queries: Different message if no index is found

[felipeelia]

Description of the Change

If the Elasticsearch server does not have any index related to the current WordPress installation, the "Failed queries" message will be different, pointing the user to sync.

This PR also adds a new method called get_indices_comparison() to the Elasticsearch class. It returns an array of two arrays: a list of present indices, and a list of missing ones.

How to test the Change

1. Enable more than one Indexable (Comments or Terms, for example)
2. Sync
3. Delete one index
4. See the old message
5. Delete all indices
6. See the new message

Changelog Entry

Changed - If no index is found, the failed queries message will be replaced with a prompt to sync

Credits

Props @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.