Don't `exit` if a renamed label wasn't found

[jean]

If we see a request to rename a non-existent label, just complain and continue with the rest of the work.

iff --git a/src/labels/cli.py b/src/labels/cli.py
index 6cb7723..55e516d 100644
--- a/src/labels/cli.py
+++ b/src/labels/cli.py
@@ -204,7 +204,7 @@ def sync_cmd(
                     f'parameter: "{local_label.name}"',
                     err=True,
                 )
-                sys.exit(1)
+                labels_to_ignore[remote_name] = local_label

I'm driving labels from another package, running it across many repos.
Some repos may not have the label that I'm renaming.
I'd like it to skip the missing label and continue.