There are a few options using the API but I haven't tested any of them.

1. You could write a scheduled task that runs more often than your emails are sent from the queue. That scheduled task would look in the email queue for all the emails that haven't been sent (status = 1 I believe) and maybe some subject text checking (subject like '%error%'). Then update the record with the subject text you want.

2. Create a global handler to check before insert the object (cms.email) and if its an event or error log, modify the subject text.

You should be able to find some help on the DevNet for creating global handlers and should also find some info on creating a scheduled task.