Wrap PL/Python SPI calls into subtransactions

Wrap PL/Python SPI calls into subtransactions

This allows the language-specific try/catch construct to catch and
handle exceptions arising from SPI calls, matching the behavior of
other PLs.

As an additional bonus you no longer get all the ugly "unrecognized
error in PLy_spi_execute_query" errors.

Jan Urbański, reviewed by Steve Singer