mediacheck: show "Error: implantisomd5 is not available" on screen

features.in/mediacheck/config.mk

@@ -3,7 +3,7 @@ use/mediacheck: use/stage2 sub/stage2@mediacheck \
 	use/grub/mediacheck.cfg use/syslinux/mediacheck.cfg
 	@$(call add_feature)
 	@if ! command -v implantisomd5 >/dev/null 2>&1; then \
-	    echo "Error: implantisomd5 is not available! Need install isomd5sum"; exit 1; \
+	    echo "Error: implantisomd5 is not available! Need install isomd5sum" >&2; exit 1; \
 	fi
 	@$(call add,POSTPROCESS_TARGETS,90mediacheck)
 else