If you really want to do this, put the update statement in a PL/QSLblock with an exception. You know the errorcode, use it in the OTHERS exception or even better use DUP_VAL_ON_INDEX exception.

You get something like:

BEGIN
UPDATE your_table
SET unique_col = new_val
WHERE condition
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE = -1
THEN
NULL;
ELSE
RAISE;
END IF;
END;

or:

BEGIN
UPDATE your_table
SET unique_col = new_val
WHERE condition
EXCEPTION
WHEN dup_val_on_index THEN
NULL:
END;