Improve exception when creating a cyclic dependency in Maniphest

Summary: Fixes T6815. This was overlooked in D9838. This could be prettier, but does the job.

Test Plan: {F327790}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6815

Differential Revision: https://secure.phabricator.com/D11937