build: --keep-going with docs build to show all errors at once

Without --keep-going, 'make html' showed a single error at a time.
With --keep-going, 'make html' shows them all in one big list.
(Either way, the build would fail if there are any warnings, as desired.)

Author: kdmccormick

docs/Makefile

@@ -42,7 +42,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) --keep-going -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."