PL/Python DO handler

Also cleaned up some redundancies between the primary error messages and the
error context in PL/Python.

Hannu Valtonen

src/pl/plpython/sql/plpython_do.sql

@@ -0,0 +1,3 @@
+DO $$ plpy.notice("This is plpythonu.") $$ LANGUAGE plpythonu;
+
+DO $$ nonsense $$ LANGUAGE plpythonu;