fix the defect in the shell scripts

when the binary is missing, it should prompt for downloading instead of
sliently move on.

cluster/common.sh

@@ -323,6 +323,9 @@ function find-tar() {
 #   KUBE_MANIFESTS_TAR
 function find-release-tars() {
   SERVER_BINARY_TAR=$(find-tar kubernetes-server-linux-amd64.tar.gz)
+  if [[ -z "${SERVER_BINARY_TAR}" ]]; then
+	  exit 1
+  fi
   if [[ "${NUM_WINDOWS_NODES}" -gt "0" ]]; then
     NODE_BINARY_TAR=$(find-tar kubernetes-node-windows-amd64.tar.gz)
   fi