Show warning when binfiles aren't available

Makefile

@@ -13,16 +13,24 @@ _site:
 critical: _site
 	node _support/critical.js
 
+ensure-bin:
+	@if [ ! -d $(npmbin) ]; then \
+		echo "---"; \
+		echo "Error: $(npmbin) not found, you may need to run '[docker-compose run --rm web] yarn install'."; \
+		echo "---"; \
+		exit 1; \
+		fi
+
 # Starts development server
-dev:
+dev: ensure-bin
 	$(npmbin)/concurrently -k -p command -c "blue,green" \
 		"make dev-webpack" \
 		"make dev-jekyll"
 
-dev-webpack:
+dev-webpack: ensure-bin
 	$(npmbin)/webpack --watch --colors -p
 
-dev-jekyll:
+dev-jekyll: ensure-bin
 	if [ -f _site ]; then \
 		bundle exec jekyll serve --safe --trace --drafts --watch --incremental --host $(HOST) --port $(PORT); \
 		else \