Throw on accessing an undeclared property

Summary: Currently, Phobject throws when trying to set an undeclared property but not when trying to get an undeclared property. It seems reasonable to add this functionality.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13493