Fix another qsprintf() straggler in "Has Open Subtasks"

Summary: See https://discourse.phabricator-community.org/t/error-message-is-not-being-logged-when-unable-to-connect-to-the-database/2201/.

Test Plan: Queried for "With Open Subtasks" and "With No Open Subtasks".

Reviewers: amckinley, joshuaspence

Reviewed By: joshuaspence

Subscribers: joshuaspence

Differential Revision: https://secure.phabricator.com/D19880