Travis: add warning when failing

.travis.yml

@@ -2,8 +2,9 @@ language: ruby
 rvm:
   - 2.5.0
 script:
-  - make test
+  - make _site
+  - if ! make test; then make test-warning; exit 16; fi
 cache:
   directories:
-  - node_modules
-  - _site
+    - node_modules
+    - _site

Makefile

@@ -35,3 +35,10 @@ test: _site
 	@grep "<script src" _site/index.html >/dev/null
 	@grep "<script src" _site/vim.html >/dev/null
 	@grep "<script src" _site/react.html >/dev/null
+
+test-warning:
+	@echo "========="
+	@echo "If your build failed at this point, it means"
+	@echo "the site failed to generate. Check the project"
+	@echo "out locally and try to find out why."
+	@echo "========="