do not fail when scanning unreadable files
do not fail when scanning unreadable files

We should just skip these files.

Some archive formats preserve the ownership and permissions of a file
from before it was archived. Some reasonable tools like `tar` give you
the option of ignoring these and writing out the permissions of the user
running the `tar` process. Some unreasonable tools like `unzip` don't
let you set this.

[#126615597]

pivotal-cf/cred-alert,pivotal-cf/cred-alert