Error from make install when sphinx isn't installed is opaque at best

You get something like this:

> [opnfv@localhost docs]$ make html
> Makefile:12: *** recipe commences before first target. Stop.

It would be good to say something about installing sphinx to stop it.

This may be need to be a bug filed in Sphinx.

Started from this thread:
https://lists.opendaylight.org/pipermail/documentation/2016-June/000757.html