Show admin notice or log message when Used-CSS subfolders do not have writing permissions

[suzoutlet]

Describe the Issue
We usually show an admin notice when WP Rocket folders, such as the used-css folder is not writable, but with the sub-folders that do not have write permission, no notice is shown.

An example case is a Used CSS subfolder getting created, but due to write permission, the Used CSS cannot be applied to the pages.

Expected behavior
Reference from @engahmeds3ed

We may need to enhance this to cover this case, or at least show log message related to it because it's confusing to save the message coming from the API response in error message when it succeeds.

Additional context

• Link to Slack discussion