Better error handling on indexing

Problem/Motivation

When embedding moderation throws an error it is not possible to know which item did throw the error. Furthermore the whole indexing process stops and there is no way to get other items indexed.

Steps to reproduce

Add an item which throws a moderation error to the indexing queue.

Proposed resolution

1) Enhance logging so the item which fails should be clearly visible in the logs (either AI log or Drupal recent messages) in order to enable users to fix it.
2) Add an option to allow skipping items which throw an error to at least get other items into the index. A simple optional try/catch in the execute-method with proper logging (see 1) of the batch might be all it needs.

API changes

Data model changes