Raise a better error for malformed git ls-tree

Summary: Ref T4159. See T4159 for discussion.

Test Plan: Faked the error and generated a reasonable error message.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4159

Differential Revision: https://secure.phabricator.com/D7744