Handle the whole of PostgreSQL errors.

In a recent test case about camelCasing column names, another kind of
error did pop up and there's really no point not to handle it correctly.
Handle camelCase column names for CSV, fix #79 again.

The previous patch didn't take into account the need to retain the case
of the PostgreSQL column names when using double-quotes in the load
command, which is now properly forwarded down in the COPY command.