Fail make docs on docs warnings

Author: Ayaz Salikhov

Makefile

@@ -138,7 +138,7 @@ install-dev-env: ## install libraries required to build images and run tests
 
 
 docs: ## build HTML documentation
-	sphinx-build docs/ docs/_build/
+	sphinx-build -W docs/ docs/_build/
 
 install-docs-env: ## install libraries required to build docs
 	@pip install -r requirements-docs.txt