fix: report unreadable folders

The CLI's walk skipped a folder or entry it couldn't read without a
word, so `--check` could pass over files it never saw. Those are now
errors, printed with their path, and the run exits 2, as it does for
a file it can't read.

A panic while formatting one file also lost every result of that
worker thread. Each file is now formatted under catch_unwind, and a
panic becomes that file's error.