How to raise error from PostgreSql SQL statement if some condition is met

I’m looking for a way to raise error from sql select if some condition is met.
Tried code below to got error shown in comment.
How to fix ?

Andrus

CREATE OR REPLACE FUNCTION "exec"(text)
RETURNS text AS
$BODY$
BEGIN
EXECUTE $1;
RETURN $1;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;

-- ERROR: syntax error at or near "raise"
-- LINE 1: raise 'test'

select exec('raise ''test'' ') where true -- in real application true is replaced by some condition