pl/python tracebacks v2

I finally got around to updating the PL/Python tracebacks patch. The
other day I was writing some very simple PL/Python code and the lack of
tracebacks is extremely annoying.

Getting a TypeError without any line information in a function with 30
lines because there's a %d instead of a %s somewhere in a logging call
can make your debugging experience very lousy.

Attached is a patch against master, with simplified error forming logic
and the traceback in the context field. If you look at the expected
regression test output, you will see that the only thing that changes is
the presense of tracebacks in context messages.

I'll update the commitfest app for the 2011-Next commitfest, but if
someone would like to pick this up and include it in the 9.1 PL/Python
revamp pack, I'd be thrilled.

Cheers,
Jan