Refuse writing to undeclared properties in workflows

Summary: I don't know how to not be strict here plus we (Arcanist developers) don't have access to user's error log.

Test Plan:
Undeclared ArcanistDiffWorkflow::$console, then:

$ arc diff

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3607