Unhelpful error message on missing variables

e.g.:

    Error loading pipeline: /home/jimmacarthur/.local/lib/python3.6/site-packages/buildstream/data/projectconfig.yaml [line 14 column 2]: Dictionary did not contain expected key 'python'

This is caused by a missing definition for the variable python in an element - python-diffoscope.bst in this case. projectconfig.yaml is I think the last place it looks for the definition of python. We should report the location of the variable reference that couldn't be found instead.