Improve finding manifest state

[kroepke]

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