Add an error key custom field to Maniphest tasks.

Summary:
The error key can be used by the error monitor to track the status of fixing
observed errors and show it on the errors page at /devadmin/errors.

Test Plan: Crossed fingers.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D14182