Return Phoenix connection for any uncaught webhooks

[SimonLab]

linked to https://github.com/dwyl/github-backup/issues/65#issuecomment-372579790

If an event is not matched the server should still return a response. Instead of just returning nil https://github.com/dwyl/github-backup/blob/7b6d7332f72e0b3ebe793e4bb0f4e7ea5894c829/lib/app_web/controllers/event_type.ex#L16 We want to return a 404 reply

conn
|> put_status(404)
|> json(%{ok: "event unknown"})

[nelsonic]

@SimonLab thanks for addressing this. 🎉