pl/python custom exceptions for SPI

Here's a patch implementing custom Python exceptions for SPI errors
mentioned in
http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
an incremental patch on top of the explicit-subxacts patch sent eariler.

Git branch for this patch:
https://github.com/wulczer/postgres/tree/custom-spi-exceptions.

What the patch does is provide a Python exception per each error defined
in utils/errcodes.h, and then raise the corresponding exception when a
SPI call fails. The parsing of errcodes.h in the Makefile is a little
grotty and would probably have to be ported to the Windows build system,
which I have no idea about.

With this patch you can do:

from plpy import spiexceptions

try:
    plpy.execute("insert into t values (4)")
catch spiexceptions.UniqueViolation:
    plpy.notice("unique constraint violation")
catch spiexceptions.NotNullViolation:
    plpy.notice("not null constraint violation")

All exceptions inherint from plpy.SPIError, so code thta just catches a
blanket SPIErorr will continue to work.

The patch lacks user-facing docs, if it gets accepted I'll write some.
Not sure if we should provide a table similar to
http://www.postgresql.org/docs/current/static/errcodes-appendix.html, or
just refer to that table and state that the rule is changing underscores
to camel case...

Also, I just realised that this patch does not really depend on the
explicit-subxacts change, but rather only on the spi-in-subxacts, so if
need be I can generate it as an incremental changeset ofer the latter
and not the former.

Cheers,
Jan