We recently wrote a python script for this exact purpose, it runs daily, consuming the WebHook API: https://developer.zendesk.com/api-reference/webhooks/webhooks-api/webhooks/

The script works in the following way:

Get all Webhooks: https://developer.zendesk.com/api-reference/webhooks/webhooks-api/webhooks/#list-webhooks
Loop over all Webhooks and get the invocations (each time it ran): https://developer.zendesk.com/api-reference/webhooks/webhooks-api/webhooks/#list-webhook-invocations
1. Use filter[status] to filter on only the error statuses
2. Use filter[from_ts] and filter[to_ts] to capture only the errors in a specific time range (i.e. since the script last ran till now)
Add each error as a message to then send in Slack in 1 go.

As the Invocation history last 7 days we run this on a daily basis and we can see the list of errors per WebHook