Parse the error codes from the text file if available

The text file was added in PG 9.1. It contains a few errors not available
in the SGML.
Add a few missing errcodes

They are used in the code but not defined in the SGML docs so the script
failed to parse them.
Parse errocodes for PG 9.3 too

There is no new errcode defined so no need to release this as a change,
only change the docs.