Another error to warning

Change-Id: Ifbad63e3eb37d087d88a39e102b4239f305d0ee3
diff --git a/core/main.mk b/core/main.mk
index 874bb0b..7bd55b8 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -659,7 +659,7 @@
   # Ensure every module listed in PRODUCT_PACKAGES gets something installed
   $(foreach m, $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_PACKAGES), \
       $(if $(strip $(ALL_MODULES.$(m).INSTALLED)),,\
-          $(error $(ALL_MODULES.$(m).MAKEFILE): Module '$(m)' in PRODUCT_PACKAGES has nothing to install!)))
+          $(warning $(ALL_MODULES.$(m).MAKEFILE): Module '$(m)' in PRODUCT_PACKAGES has nothing to install!)))
 endif
 
 # Install all of the host modules