chg: dev: be more explicit when failing to merge yaml

bin/compose-core

@@ -298,7 +298,15 @@ merge_cli(*(yaml.load(f, Loader=MySafeLoader) for f in sys.argv[1:]))
 EOF
     fi
 
-    python "$state_tmpdir/merge_yaml_str.py" "$@"
+    if ! python "$state_tmpdir/merge_yaml_str.py" "$@"; then
+        err "Failed to merge yaml strings:"
+        local s
+        for s in "$@"; do
+            printf " - \n"
+            printf "%s\n" "$s" | prefix "  ${GRAY}|$NORMAL "
+        done >&2
+        return 1
+    fi
 }
 export -f merge_yaml_str