Asper my knowledge it is not necessary to writye all the errors to alert logs . for certail messages to come in alert log we need to set the appropriate parameter in init.ora file .
pls let me know which Oracle failed jobs u are talking about . is this a user defined job or default background process ??
I don't think whether User defined jobs are ever shown in alert log file .
i just remember that there are some initialisation parameters, which will let DBMS_jobs failed display in alert log . ( by setting SNP process and Job queing ) .