Before this commit, if Walker.next errored with e.g. `error.AccessDenied` and the caller did something like `while (true) { walker.next() catch continue; }`, then the directory that errored with AccessDenied would be continually iterated in each `next` call and error every time with AccessDenied.

After this commit, the directory that errored will be popped off the stack before the error is returned, meaning that in the subsequent `next` call, it won't be retried and the Walker will continue with whatever directories remain on its stack.

For a real example, before this commit, walking `/proc/` on my system would infinitely loop due to repeated AccessDenied errors on the same directory. After this commit, I am able to walk `/proc/` on my system fully (skipping over any directories that are unable to be iterated).