Refactored custom PDO_pgsql methods to trigger errors/exceptions

Refactored custom PDO_pgsql methods to trigger errors/exceptions

BC Break: the custom methods were previously just return false on
failure. Now they throw an exception with a proper error message.
An hopefully welcome improvement, but some application might be
depending on the old behaviour. FWIW the PDO::pgsqlCopy* methods
are not documented, even though they are available since 5.3.x.