Add separate exception for when the repository clone is unreadable.

Summary: Show a more specific exception when the local clone cannot be read because of permission issues.

Test Plan: Create a repository in an unreadable location and check for the right exception.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2368

Differential Revision: https://secure.phabricator.com/D4868