Fix failure to think clearly about encoding conversion errors in COPY.

Fix failure to think clearly about encoding conversion errors in COPY.
We can't regurgitate the unconverted string as I first thought, because
the elog.c mechanisms will assume the error message data is in the server
encoding and attempt a reverse conversion.  Eventually it might be worth
providing a short-circuit path to support this, but for now the simplest
solution is to abandon trying to report back the line contents after a
conversion failure.  Per bug report from Sil Lee, 27-Oct-2004.