Bug 1421241 - Docs: Improve error shown when Sphinx not installed

docs/Makefile

@@ -6,7 +6,7 @@ BUILDDIR      = _build
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
-$(error The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the '$(SPHINXBUILD)' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/)
+$(error Sphinx not found. Run 'pip install -r requirements/docs.txt' first.)
 endif
 
 .PHONY: help clean html livehtml