Improve finding manifest state

If the tool isn't called in the directory which contains the `.graylog-project-manifest-state` file (most often this happens when being called from the `manifests` directly itself, the error message isn't all that helpful.

Ideally we'd just walk the path upwards to find the file.

Current error is just:

```
Unable to read manifest state from .graylog-project-manifest-state: open .graylog-project-manifest-state: no such file or directory
```