Getting more information about errorcodes such as when these error1 happen

Good morning,

In my plpgsql functions I use "exception when..." to catch possible 
exceptions of my data. I found postgresql error code track functions are 
very helpful.  In my functions, I will not catch all error codes, but 
only the ones applying to my data operation .

I found errorcodes info here:
http://www.postgresql.org/docs/8.1/static/errcodes-appendix.html

But I am afraid that I could not imagine when and under what possible 
circumstances some errorcodes may happen just by their names such as : 
locator_exception, invalid_grantor,  active_sql_transaction , and so on.

I tried to search the online docs in order to get more info such as when 
will errorcode X happens. But I could not find it.

Could someone tell me some links that I can find more information about 
these errorcodes please?

Thanks alot,
Ying