User Defined Functions Errors

How to I return an error from inside a user defined function? For
example the following:

CREATE or REPLACE FUNCTION drop_node (integer) RETURNS text
AS '

DECLARE

mleft INTEGER;
mright INTEGER;

BEGIN

-- Check if the desired node exists
SELECT lft, rgt FROM structure WHERE id = $1 INTO mleft, mright;

IF mleft IS NULL THEN
RETURN ''No entry found with an id of ''||$2;
END IF;

-- Drop the node and its subtree
DELETE FROM structure WHERE lft >= mleft AND rgt <= mright;

-- Close the gap
UPDATE structure SET rgt = rgt - (mright - mleft + 1) WHERE rgt > mright;
UPDATE structure SET lft = lft - (mright - mleft + 1) WHERE lft > mleft;

RETURN ''ok'';

END;
'
LANGUAGE 'plpgsql';

Should be terminated with an error @ RETURN "No Entry found"; instead
of returning the error text.

best regards and thanks,

Alex
--
The mind is essential -- http://essentialmind.com/