pgsql: Use errcontext mechanism in PL/Python Error messages from

Log Message:
-----------
Use errcontext mechanism in PL/Python

Error messages from PL/Python now always mention the function name in the
CONTEXT: field. This also obsoletes the few places that tried to do the
same manually.

Regression test files are updated to work with Python 2.4-2.6. I don't have
access to older versions right now.

Modified Files:
--------------
pgsql/src/pl/plpython:
plpython.c (r1.122 -> r1.123)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.122&r2=1.123)
pgsql/src/pl/plpython/expected:
plpython_error.out (r1.5 -> r1.6)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_error.out?r1=1.5&r2=1.6)
plpython_error_3.out (r1.2 -> r1.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_error_3.out?r1=1.2&r2=1.3)
plpython_function.out (r1.12 -> r1.13)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_function.out?r1=1.12&r2=1.13)
plpython_test.out (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_test.out?r1=1.8&r2=1.9)