Add setHint/getHint to PhutilLockException for recent lock changes

Summary: Ref T13202. See PHI889. See D19702. A small part of that change affected libphutil, but got lost in the shuffle with all the Arcanist/wilds stuff in my local workspace.

Test Plan: See D19702.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13202

Differential Revision: https://secure.phabricator.com/D19720