PL/Python: Adjust the regression tests for Python 3.4

The error test case in the plpython_do test resulted in a slightly
different error message with Python 3.4.  So pick a different way to
test it that avoids that and is perhaps also a bit clearer.

src/pl/plpython/sql/plpython_do.sql

@@ -2,4 +2,4 @@ DO $$ plpy.notice("This is plpythonu.") $$ LANGUAGE plpythonu;
 
 DO $$ plpy.notice("This is plpython2u.") $$ LANGUAGE plpython2u;
 
-DO $$ nonsense $$ LANGUAGE plpythonu;
+DO $$ raise Exception("error test") $$ LANGUAGE plpythonu;