Models for thing-not-found messages

[dangunter]

As we move towards having parts of IDAES installed optionally, I thought it would be useful to see some good examples for messages to the user when something that is needed is not found. I will start with the one you get when you try to build the Sphinx documentation from the batch file without first activating an environment that has Sphinx installed (not that anyone I know would be silly enough to try that):

The 'sphinx-build' command was not found. Make sure you have Sphinx
installed, then set the SPHINXBUILD environment variable to point
to the full path of the 'sphinx-build' executable. Alternatively you
may add the Sphinx directory to PATH.

If you don't have Sphinx installed, grab it from
http://sphinx-doc.org/