Hetzner: search for not_found code explicitly before returning ErrInstanceNotFound

[davidhiendl]

When a hetzner cloud instance is deleted externally (not by the autoscaler, for example via GUI/API) the autoscaler fails to correctly remove the instance. This is due to the fact that it does not return ErrInstanceNotFound but a generic one.

Example log:

{"image":"ubuntu-20.04","level":"debug","msg":"deleting instance","name":"agent-QpBgrGhL","region":"unknown","size":"cpx11","time":"2022-09-22T13:57:36Z"}
{"error":"server with ID '24078837' not found (not_found)","image":"ubuntu-20.04","level":"error","msg":"deleting instance failed","name":"agent-QpBgrGhL","region":"unknown","size":"cpx11","time":"2022-09-22T13:57:37Z"}
{"error":"server with ID '24078837' not found (not_found)","level":"error","msg":"cannot destroy server","server":"agent-QpBgrGhL","state":"error","time":"2022-09-22T13:57:37Z"}

As you can see the hetzner responsed with code=not_found but the implementation returns a generic error as indicated by the log message "cannot destroy server" (reaper.go#107)

Instead of relying on the generic error response here which seems to have changed it will now check for the string not_found in the hetzner api response and if found return ErrInstanceNotFound allowing the instance to be deleted from the autoscaler.

[ShruthiHubb]

Hi @davidhiendl Thanks for sending the proposal we appreciate it 🚀

We usually review PRs every Tuesday but I will keep you up to date if anything changes.