Failure to visit cgroups hierarchy due to missing file.

[tobz]

Saw this error log in staging complaining about a missing file when trying to visit the cgroups hierarchy:

yyyy-MM-dd HH:mm:ss UTC | DATAPLANE | ERROR | (lib/saluki-env/src/workload/helpers/* | error:No such file or directory (os error 2) | Failed to visit cgroups hierarchy.

This error log can crop up for any underlying error we encounter when traversing the cgroup hierarchy, which potentially means it's similar to #498 in that we found a cgroup controller file for a container, but the container exited before we actually read the file? Something along those lines.

It might just be that we want to use more customized error messages in visit_subdirectories to differentiate where in the traversal it failed, and on what path the failure occurred.