fix: check if entries in session_dir are readable files

[jedrzejboczar]

Closes #64.

[josh-nz]

I added a similar check in my PR, but for the existence of a file, to make a nicer error message. I'll update my commit 984b34a to use your code.

[josh-nz]

Ah, I see your change was in the list function, mine was in the load function. I'll add a review to my change to discuss this further. Disregard my above comment.