Bug: when deleting a folder that contains a file we can't delete, we get an uninformative message

To reproduce:
1. Create folder `/path_to_my/folder`
2. Create nested_folder `/path_to_my/folder/nested_folder`
3. Create a file inside nested_folder `/path_to_my_folder/folder/nested_folder/file.txt`
4. Change the permissions of `nested_folder` to be read only.
5. Start diskonaut and give it `/path_to_my` as its path (`diskonaut /path_to_my`).
6. Try to delete folder.
7. See that we get an uninformative error message and are unsure what happened.

In this scenario, the message should read something like "Permission denied deleting /path_to_my/folder/nested_folder".

Start at the diskonaut CLI entry point and reproduce the deletion failure using the numbered steps, including a read-only nested_folder. Trace the folder-deletion error handling; done means the failure reports permission denial and identifies /path_to_my/folder/nested_folder.

Written by the indexing model from the issue text.