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