Fail on non-existing hashes for fetching

mk/fetch.mk

@@ -81,6 +81,7 @@ $(1):
 			git clone --progress $${PKG_SITES} $${PKG_NAME}-$${PKG_VERSION} $(DL_TRACE); \
 			if [ $$$$? -ne 0 ]; then echo "git clone error"; exit 1; fi; \
 			(cd $${PKG_NAME}-$${PKG_VERSION}; git checkout $${PKG_VERSION}) $(DL_TRACE); \
+			if [ $$$$? -ne 0 ]; then echo "git checkout error"; exit 1; fi; \
 			;; \
 		  esac ;\
 		else \