fs-mistrust errors sometimes result in misleading messages

The error message in #1466 (closed) is

./target/quicktest/arti: error: bootstrap: tor: cache access problem: Unable to bootstrap a working directory: Bad permissions in cache directory: File or directory ${HOME}/.cache/arti/dir_blobs/con_microdesc_sha3-256-c18016c7c40833ab36668dd2e25feb2e9c97709ceb19882c1159075f8e952ec1 not found

The words "Bad permissions" are a lie.

The root cause is probably that that string is being applied, generally, to fs_mistrust::Error. But fs-mistrust errors can occur for all sorts of reasons, so that message is not appropriate. Indeed even "error checking permissions" may be too misleading.