setup should show a warning when documentation cannot be generated due to missing sphinx

I tried running setup.py without sphinx installed and I get

$ ./setup.py build
running build
running build_py
creating build
creating build/lib
copying filestore.py -> build/lib
running build_scripts
creating build/scripts-3.2
copying and adjusting dmediasum -> build/scripts-3.2
changing mode of build/scripts-3.2/dmediasum from 664 to 775

There isn't any mention that documentation generation cannot go forward.

Now after I install sphinx, I get
$ ./setup.py build
running build
running build_py
running build_scripts
Making output directory...
Running Sphinx v1.0.7
loading pickled environment... not yet created
building [html]: targets for 4 source files that are out of date
updating environment: 4 added, 0 changed, 0 removed
reading sources... [100%] protocol
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] protocol
writing additional files... genindex search
copying static files... done
dumping search index... done
dumping object inventory... done
build succeeded.

Please add a warning message like
"WARNING: Documentation generation failed because sphinx is not installed".

This looks trivial but is a boon in disguise for packagers who will have no clue why documentation is not being generated.

Same holds for microfiber too