Halt build on YAML error without jinja2

This fixes the issue described in both #247 and #137 by detecting parse errors related to "{{" inside a YAML file.  The major reason for those to be there is when doing template work.
Sure -- I'll have to add a `sys.exit(1)` here as well so it halts execution.  I'm also going to add some TODOs around using exceptions here so this code can be used in the context of a library.