pkgtree: add os.IsPermission check for walkFn err arg

[jmank88]

What does this do / why do we need it?

This PR adds a check for the incoming err argument of this walkFn, in response to a recent change on tip.

Which issue(s) does this PR fix?

Fixes #1220

[sdboyer]

kinda obnoxious that we now have to have the os.IsPermission() check in multiple places 😢

let's included some comments on this, and the call to it later down, in order to clarify why the check appears twice.

[jmank88]

What did you have in mind? I can qualify the old comment as only applying to <=1.9, but the new code at the top seems self explanatory.

[sdboyer]

heh, i literally didn't notice that we already had a detailed comment written there 😛

but the versioned addition to it is 👍