Error on undefined labels.

docs/Makefile

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -W --keep-going
 SPHINXBUILD   = python -msphinx
 SPHINXPROJ    = JupyterLab
 SOURCEDIR     = source
@@ -17,4 +17,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

docs/environment.yml

@@ -3,6 +3,6 @@ channels:
   - conda-forge
 dependencies:
 - python=3.5
-- sphinx
+- sphinx>=1.8
 - sphinx_rtd_theme
 - recommonmark