Revert "resave main() returns nothing to indicate no error"

This reverts commit 448016e.

src/ome2024_ngff_challenge/resave.py

@@ -406,9 +406,10 @@ def write(
         config.zr_write_text(filename, text)
 
 
-def main(ns: argparse.Namespace) -> None:
+def main(ns: argparse.Namespace) -> int:
     """
-    If no images are converted, raises SystemExit.
+    If no images are converted, raises
+    SystemExit. Otherwise, return the number of images.
     """
 
     converted: int = 0
@@ -549,6 +550,7 @@ def main(ns: argparse.Namespace) -> None:
 
     if converted == 0:
         raise SystemExit(1)
+    return converted
 
 
 def cli(subparsers: argparse._SubParsersAction):