store/darwin: wrap ReadDir error on backup dir
store/darwin: wrap ReadDir error on backup dir

Otherwise errors like 'ERROR: readdirent: invalid argument' are
returned which help no one. I ran into this because I was switching
from the darwin-v100 and master branches and running `backup` commands
on both.. Silly me.

No one should have the old backup format so I'm not worried about a
migration or better error handling.

Issue: https://github.com/adamdecaf/cert-manage/issues/9

adamdecaf/cert-manage,adamdecaf/cert-manage