Before this change we counted the final summary error as an error,
producing confusing log messages like:
Failed to check with 54 errors: last error was: 53 differences found
This change marks the summary error as already being counted, so the
error message becomes:
Failed to check with 53 errors: last error was: 53 differences found
This change also returns a listing failure in preference to a summary error.
See: https://forum.rclone.org/t/slow-checksum-validation/19763/22
4e2b5389d7