When client_encoding is dispatch to QE, error messages generated in QEs were
converted to client_encoding, but QD assumed that they were in server encoding,
it will leads to corruption.

This is fixed in 6X in a6c9b4, but this skips the gpcopy changes since 5X
doesn't support syntax 'COPY...ENCODING'.

Fix issue: https://github.com/greenplum-db/gpdb/issues/10815