Increase the severity of "use of undefined constant" to exception

Summary: PHP emits a warning about `if (XYZ)` (e.g., missing `$`) and similar and treats it as `"XYZ"`. We //never// mean `"XYZ"`. In libphutil code, this is always a severe error, and should abort execution.

Test Plan:
Added one to the homepage and got this:

{F36493}

Reviewers: ljalonen

Reviewed By: ljalonen

CC: aran

Differential Revision: https://secure.phabricator.com/D5393