Add Mailgun webhook for check failures

[adriensalz]

In September, 4 outbound checks failed to generate/send due to an internal Mailgun error:

{ "event_type": { "enabled_for_test": true, "id": "check.failed", "resource": "checks", "object": "event_type" }, "reference_id": "chk_45e70ec79e53a31a", "date_created": "2024-09-24T16:21:35.158Z", "id": "evt_18e29f09b3ee94fd", "body": { "id": "chk_45e70ec79e53a31a", "status": "error", "reason_type": "failed rendering", "reason_subtype": "ERROR: Failed to convert the document to PDF/X.\n[Error 0x40000001] An internal error occurred.\n", "resource_type": "check", "remediation": "try re-render", "metadata": {} }, "object": "event" }

https://www.muckrock.com/foi/spokane-71/courtney-corrinna-holden-172524/

https://www.muckrock.com/foi/albuquerque-343/nikki-turrietta-161114/

https://www.muckrock.com/foi/phoenix-94/caelee-love-light-160142/

https://www.muckrock.com/foi/ferguson-8837/police-data-collection-request-2017-2022-172688/

It wasn't obvious to staff that something had gone wrong until a month later, when a user alerted Support to an issue with their check.

https://muckrock.zendesk.com/agent/tickets/134217

On the request page, the check hadn't moved past Step 1, Created, since September.

This doesn't seem to be a widespread issue. It looks like these 4 checks are the only time we've had something fail to send in our time using Mailgun:

Still, since user money is involved, we want to set up a webhook to catch future errors ASAP and resend any impacted checks.