Return meaningful response status for webhook error

[devnook]

Instead of always returning an HTTP 200 in /approvals handler, we should propagate some errors that come from handling the webhook and use those to trigger an error response whose body contains details about the failure.

https://docs.github.com/en/github-ae@latest/rest/guides/best-practices-for-integrators#use-appropriate-http-status-codes-when-responding-to-github makes it sound like including errors about the failure in the response body could be helpful in debugging.

context: #12