Throw instead of E_NOTICE from AASTNode::getChildByIndex(0) with no children

Summary: It should also improve performance if the child is not found.

Test Plan: Call it with 0 on node without children.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2826