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

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/dbbfd4111891c831a9df6b7a0df600b3f76c52be

commit dbbfd4111891c831a9df6b7a0df600b3f76c52be
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Mon Dec 12 11:37:23 2016 +0100

    download.mk: use $(error) instead of a regular shell error
    
    Useful for further rework
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 include/download.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/download.mk b/include/download.mk
index 9fbc3fb..b234b1b 100644
--- a/include/download.mk
+++ b/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