Summary:
If `path_search()` returns that CMake is not available, raise an Exception and
fail the build.  This makes the failure somewhat easier to to identify.
Without this the code would continue and would try to invoke `subprocess` with
a value of `None` in the command argument list.  This error is slightly harder
to debug, since it isn't clear which command argument or environment variable
is not a string.

Reviewed By: chadaustin

Differential Revision: D16354623

fbshipit-source-id: be972b02cb13bc70db0f867da70e5bf4c6cec46d