Raise exception for at least one type of syntax errors

Every so often, I create a yaml file with something like:

- job:
  name: "test"

This previously exploded with an obscure error message, now it raises
an exception with a hopefully less obscure error.

Change-Id: Ie9e402a9ec5afa8dac069a6cae5710eabbc0e665
Signed-off-by: Tollef Fog Heen <tfheen@varnish-software.com>
Reviewed-on: https://review.openstack.org/20322
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins