Support "OR condition ..." in plpgsql EXCEPTION clauses to make the syntax

Support "OR condition ..." in plpgsql EXCEPTION clauses to make the syntax
more nearly Oracle-equivalent.  Allow matching by category as well as
specific error code.  Document the set of available condition names
(or more accurately, synchronize it with the existing documentation).  In
passing, update errcodes.sgml to include codes added during 7.5 development.