Notify items if an error occurs in bulk indexer

[ansssu]

Propagate errors on items. Fix #179

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

💚 CLA has been signed

[TCheruy]

@Anaethelion (as last main contributor) would you mind taking a look at this PR when you get a chance? #179 has been going on for a long time and prevents from using the bulk indexer in many production use-cases where visibility into failed writes is needed.

[prodsecmachine]

✅ Snyk checks have passed. No issues have been found so far.

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[MattDevy]

Hi all, I've added some tests to cover the new behaviour, with a slight refactor to make the code more DRY and a minor bug fix. This will be sorted soon, thank you for your patience and for your contribution!