Don't crash on existing bucket.

main.py

@@ -196,7 +196,9 @@ def single_impl(item_directory: pathlib.Path, ia_collection: str, ia_item_title:
         def make_bucket():
             logging.info("Attempting to make bucket...")
             if client.bucket_exists(bucket_name=bucket_name):
-                raise Exception("Bucket already exists!")
+                # If bucket already exists a previous attempt was aborted.
+                logging.warning("Bucket already exists!")
+                return
             client.make_bucket(bucket_name=bucket_name)
 
         retry_failures(make_bucket, "Failed to make bucket")