#715: [Makefile] Wipe node_modules if not properly installed

diff --git a/Makefile b/Makefile
index b8d12ef..906bfa0 100644
--- a/Makefile
+++ b/Makefile
@@ -140,6 +140,7 @@
 	$(SED_IN_PLACE) s/{VERSION}/v$(VERSION)/ $@
 
 $(NODE_MODULES_INSTALLED): package.json
+	test -e $(NODE_MODULES_INSTALLED) || rm -rf ./node_modules/ # robust against previous botched npm install
 	NODE_ENV=development npm install
 	touch $(NODE_MODULES_INSTALLED)