"Indexer failures" page not helpful

The information displayed on the "Indexer failures" page is not helpful for identifying the error.

## Expected Behavior
Provide information of the source of the message that caused the index failure, e.g. IP, source (hostname), message body, etc.

## Current Behavior
Currently "Index" and "Letter ID" are shown. Because the faulty message is not stored in Elasticsearch (or anywhere else), those two fields are completely pointless.

## Possible Solution
Provide information of the source of the message that caused the index failure, e.g. IP, source (hostname), message body, etc.

## Steps to Reproduce (for bugs)
Provoke _any_ index failure. Check if the information on the "Indexer failures" page is helpful to track down the error.

## Context
I sometimes get index failures, e.g. because of misconfigured hosts that send fields that are reserved in Elasticsearch (e.g. "_version"). At the moment there is no easy way to track down the root cause. As a workaround I use Pipelines/Rules to write more debug info into the logs.

## Your Environment
* Graylog Version: 4.0.5
* Java Version: Open JDK 8
* Elasticsearch Version: 7.8.1
* MongoDB Version: 3.6
* Operating System: Ubuntu 20.04LTS
* Browser version: Chrome