Augment test coverage in PL/Python, especially for error conditions.

src/pl/plpython/expected/plpython_params.out

src/pl/plpython/expected/plpython_record.out

src/pl/plpython/expected/plpython_schema.out

src/pl/plpython/expected/plpython_setof.out

src/pl/plpython/expected/plpython_test.out

src/pl/plpython/expected/plpython_trigger.out

src/pl/plpython/plpython.c

src/pl/plpython/sql/plpython_params.sql

src/pl/plpython/sql/plpython_record.sql

src/pl/plpython/sql/plpython_schema.sql

src/pl/plpython/sql/plpython_setof.sql

src/pl/plpython/sql/plpython_spi.sql

src/pl/plpython/sql/plpython_test.sql

src/pl/plpython/sql/plpython_trigger.sql