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
This is preparation for getting the Accounting to check the context,
buf first we need to get it in place. Since this is one of those
changes that makes lots of noise, this is in a seperate commit.
