fix: name --fetch when an original is not cached

The export failed with 'absent and fetching is off'. It now names the cache directory it looked in and the flag that downloads the file.

scripts/export_native.py

@@ -335,7 +335,9 @@ def collect_originals(
             elif path.exists():
                 payload = path.read_bytes()
             else:
-                missing.append(f"{relative}: absent and fetching is off")
+                missing.append(
+                    f"{relative}: not in {root}; rerun with --fetch to download it"
+                )
                 continue
             if archive:
                 originals.update(_read_archive(unpack, payload))