feat: return meaningful status codes

[SilasPeters]

Currently, when a webhook is called to run a systemd service, you simply get an HTTP 200 when the service has been started.

This issue suggests we optionally (as defined in the config) await the exit code of the service, and return a 200 or 500 depending on the exit code.
An example of why this is useful, is for github workflows. A workflow will await full deployment and can test it afterwards. If deployment failed, the workflow fails. If the workflow completed succesfully, you can expect to immediatly see the results. Full deployment experience!