User's exception plpgsql

Hello

Per small recent discussion I corrected patch user's exception.

diff: User can choise any sqlstate (without class U0, which I reserve as
range for default values sqlstates - if user don't spec sqlstate, is
used value from this range). There is only basic changes in documentation
and needs enhancing. I am not able to do (I am sorry, my english is poor).

Note: patch don't create deep changes in plpgsql core. Only enhance stmts
DECLARE, RAISE and EXCEPTION condition.

Next ToDo (needs discussion):
  + Optional message in raise stmt for user's or system exception
      raise exception division_by_zero;
  + Possibility rethrown exception
      raise;

Regards
Pavel Stehule