Improve exception usage in PL/Python

Improve exception usage in PL/Python

Use the built-in TypeError, not SPIError, for errors having to do with
argument counts or types.  Use SPIError, not simply plpy.Error, for
errors in PLy_spi_execute_plan.  Finally, do not set a Python
exception if PyArg_ParseTuple failed, as it already sets the correct
exception.

Jan Urbański