Use PhutilProxyException to disambiguate work queue failures

Summary: Fixes T2569. This is the other common exception source which is ambiguous. List the task ID explicitly to make debugging easier.

Test Plan: {F51268}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2569

Differential Revision: https://secure.phabricator.com/D6549