chg: dev: be more explicit on error message. !minor

bin/compose

@@ -1951,7 +1951,7 @@ launch_docker_compose() {
     fi
 
     if [ -z "$(echo $(cat "$docker_compose_dir/docker-compose.yml"))" ]; then
-        die "Unexpected empty docker-compose."
+        die "Generated 'docker-compose.yml' is unexpectedly empty. Check your charm availability in '$CHARM_STORE'."
     fi
 
     ## XXXvlab: could be more specific and only link the needed charms