image.in, build-vm: handle qemu-img absence properly

This isn't a warning cause, this is a error cause!

features.in/build-vm/lib/90-build-vm.mk

@@ -39,7 +39,8 @@ convert-image: prepare-image
 	*) VM_FORMAT="$(IMAGE_TYPE)"; \
 	esac; \
 	if ! type -t qemu-img >&/dev/null; then \
-		echo "** warning: qemu-img not available" >&2; \
+		echo "** error: qemu-img not available" >&2; \
+		exit 1; \
 	else \
 		qemu-img convert -O "$$VM_FORMAT" \
 			"$(VM_RAWDISK)" "$(IMAGE_OUTPATH)"; \

image.in/Makefile

@@ -80,6 +80,10 @@ imagedir:
 
 postprocess: | $(addprefix postprocess-,$(sort $(POSTPROCESS_TARGETS)))
 	@OUTSIZE="`ls -lh "$(IMAGE_OUTPATH)" | cut -f5 -d' '`"; \
+	if [ ! -n "$$OUTSIZE" ]; then \
+		echo "** error: $(IMAGE_OUTPATH) missing" >&2; \
+		exit 1; \
+	fi; \
 	echo "** image: $(IMAGE_OUTPATH) [$$OUTSIZE]" >&2 && \
 	echo "IMAGE_OUTPATH = $(IMAGE_OUTPATH)" && \
 	echo "IMAGE_OUTFILE = $(IMAGE_OUTFILE)" && \