Fail explicitly if curl is missing in contrib/download-frozen-image.sh

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

Dockerfile.simple

@@ -12,6 +12,7 @@ FROM debian:jessie
 # https://github.com/docker/docker/blob/master/project/PACKAGERS.md#runtime-dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
 		btrfs-tools \
+		curl \
 		gcc \
 		git \
 		golang \

contrib/download-frozen-image.sh

@@ -7,6 +7,11 @@ set -e
 # debian                           latest              f6fab3b798be        10 weeks ago        85.1 MB
 # debian                           latest              f6fab3b798be3174f45aa1eb731f8182705555f89c9026d8c1ef230cbf8301dd   10 weeks ago        85.1 MB
 
+if ! command -v curl &> /dev/null; then
+	echo >&2 'error: "curl" not found!'
+	exit 1
+fi
+
 usage() {
 	echo "usage: $0 dir image[:tag][@image-id] ..."
 	echo "   ie: $0 /tmp/hello-world hello-world"