Run on HEAD and 9.2.

This is currently the same as the condition name, so it doesn't add
any value, only clutter.

src/pl/plpgsql/src/plerrcodes.h, src/include/utils/errcodes.h, and a
big chunk of errcodes.sgml are now automatically generated from a single
file, src/backend/utils/errcodes.txt.

Jan Urbański, reviewed by Tom Lane.