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