download.mk: use $(error) instead of a regular shell error

Useful for further rework

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/download.mk

@@ -36,7 +36,7 @@ endef
 dl_pack/bz2=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -cjf $(1) $(2)
 dl_pack/gz=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -c $(2) | gzip -nc > $(1)
 dl_pack/xz=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -c $(2) | xz -zc -7e > $(1)
-dl_pack/unknown=echo "ERROR: Unknown pack format for file $(1)"; false
+dl_pack/unknown=$(error ERROR: Unknown pack format for file $(1))
 define dl_pack
 	$(if $(dl_pack/$(call ext,$(1))),$(dl_pack/$(call ext,$(1))),$(dl_pack/unknown))
 endef