fix up links to point to the right spot

Summary: The links currently 404 and redirect to `https://code.facebook.com`. Doh.

Test Plan: Inspection

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D1505678
Don't enter poison state if the dir does not allow access

Addresses https://github.com/facebook/buck/issues/183
switch to a whitelist of poison-worthy errors

Summary: There are other errnos possible here, like `ENOTDIR` or `ENOFOLLOWSYMLINK`. It's better to switch to a whitelist of errors we know we can't recover from.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D1510893
Merge pull request #1 from facebook/master

pulls stuff from origin repo