fix FilesCacheMode validator

(cherry picked from commit 0f923c8c4a03016a8b8159dcafec2ca8860e8a49)

src/borg/helpers/parseformat.py

@@ -140,11 +140,13 @@ def FilesCacheMode(s):
     VALID_MODES = ('cis', 'ims', 'cs', 'ms', 'cr', 'mr', 'd', 's')  # letters in alpha order
     entries = set(s.strip().split(','))
     if not entries <= set(ENTRIES_MAP):
-        raise ValueError('cache mode must be a comma-separated list of: %s' % ','.join(sorted(ENTRIES_MAP)))
+        raise argparse.ArgumentTypeError(
+            'cache mode must be a comma-separated list of: %s' % ','.join(sorted(ENTRIES_MAP))
+        )
     short_entries = {ENTRIES_MAP[entry] for entry in entries}
     mode = ''.join(sorted(short_entries))
     if mode not in VALID_MODES:
-        raise ValueError('cache mode short must be one of: %s' % ','.join(VALID_MODES))
+        raise argparse.ArgumentTypeError('cache mode short must be one of: %s' % ','.join(VALID_MODES))
     return mode