Error if we don't detect an hhconfig when running Hack code

Summary: Lots of folks are "running Hack code" but are utterly unaware
of the typechecker -- and so are accidentally running type-incorrect
code. It's easy enough to detect if you cannot possibly be running the
typechecker, if there is no `.hhconfig` file, so raise a fatal error in
that case.

Reviewed By: @elgenie

Differential Revision: D1407258