Expired cert for content-signature-2.cdn.mozilla.net causes heartbeat to fail to render

[sciurus]

The normandy heartbeat is intended to return a JSON response with details about any failed checks. On Saturday we discovered that if the certificate for the CDN hosting the x5u signatures for an action (content-signature-2.cdn.mozilla.net) is expired, the heartbeat does not properly report that but instead just fails with a generic error page. Presumably the same behavior occurs for the any other checks that fetch the signatures as well.

Normandy should catch and report the SSLCertVerificationError exception.

https://sentry.prod.mozaws.net/operations/normandy-prod/issues/7190941/