Fail helpfully if no elements are specified

Instead of charging ahead and failing because we lack dib-run-parts we
can fail in this case with a useful error message.

Change-Id: Ie10176b5a7019a0385796984ebbaf7742876925a

bin/disk-image-create

@@ -142,6 +142,11 @@ if [ "$IS_RAMDISK" == "1" ]; then
   source $_LIB/ramdisk-functions
 fi
 
+# If no elements are specified theres no way we can succeed
+if [ -z "$@" ]; then
+    echo "ERROR: At least one distribution root element must be specified"
+    exit 1
+fi
 arg_to_elements "$@"
 
 if [ "${#IMAGE_TYPES[@]}" = "1" ]; then