Exception names doc improvement

I think we should add another column to this table:

http://developer.postgresql.org/docs/postgres/errcodes-appendix.html

And explicitly list out all the exception names.

I think that would be clearer and easier than having a paragraph they 
need to read that says 'just use underscores'...

Also, I wonder if we really need to say 'PostgreSQL8.0.0beta1' on that 
page...

Chris