[Feature Request] Add processing treatment for emails that fail to get processed.

Description

Currently, when an email is successfully processed, it is possible to either move, label, flag, or trash the email. However, when an email is unsuccessfully processed, nothing happens in the email client.

It's possible to see which email/attachments failed in the paperless UI, but ideally, I would like to be able to at least move/flag/label an email if it was unable to be successfully processed by paperless-ngx. Even better, it would be even better to be able to do different things based on different errors: ie. duplicate attachment adds one label, generic processing error adds another label etc.

Other

This is something that I would be willing to contribute if there is enough interest :)