column already exist (ERROR replaced by warning)

In my script, i do an ALTER TABLE l_table ADD next_h varchar(64) not null;

i get after the first execution !
column name "next_h" already exists in table "l_table"

My problem is that the value returned by psql is 0, bu for me it's a warning,
not an error.(i want to obtain a 1 in this case)

Is it possible to disting the two types of error ?

thanks,

Olivier chapiteau