pgsql: Adjust phrasing of complaints about multi-byte COPY delimiter

Log Message:
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Adjust phrasing of complaints about multi-byte COPY delimiter strings.
Per pgsql-hackers discussion of 2009-02-17.

Modified Files:
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pgsql/src/backend/commands:
copy.c (r1.305 -> r1.306)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c?r1=1.305&r2=1.306)