Upgrade access to undefined property from log to exception

Summary: Just like accessing an undefined variable, this always indicates a serious error in the Phabricator codebase.

Test Plan: Ran a script which accesses an undefined property, got an exception.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3779