fix SortBySpec validator

(cherry picked from commit 05bf29f5041078d4e5d7e83e9223ec9ccdb2823d)

src/borg/helpers/parseformat.py

@@ -227,7 +227,7 @@ def SortBySpec(text):
     from .manifest import AI_HUMAN_SORT_KEYS
     for token in text.split(','):
         if token not in AI_HUMAN_SORT_KEYS:
-            raise ValueError('Invalid sort key: %s' % token)
+            raise argparse.ArgumentTypeError('Invalid sort key: %s' % token)
     return text.replace('timestamp', 'ts')