Use PhutilProxyException to disambiguate repository pull errors

Summary: Ref T2569. When repository pulls fail, they don't necessarily list identifying information about which repository failed. Use a proxy exception to list that information.

Test Plan: {F51267}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2569

Differential Revision: https://secure.phabricator.com/D6548