In order to prevent extreme condition the patch also add a new GUC variable called copy_max_error_limit that control the amount of error to swallow before start to error and new failed record file options for copy to write a failed record so the user can examine it.
Why should this be a GUC rather than a COPY option?
In fact, try doing all of this by adding more options to COPY rather than new syntax.
COPY ... WITH (ignore_conflicts 1000, ignore_logfile '...')
It kind of stinks to use a log file written by the server as the dup-reporting channel though. That would have to be superuser-only.