First of all I don't like using workflow task to handle IDoc errors because they have the tendency to fail in large numbers (if they fail) in which case the user is flooded with unnecessary information. I would suggest using a report to inform the users of errors (one mail for all errors of the day).
By the way you can use this option in the extended workflow notification which will lead to less mails (although still a lot of workflow tasks).
lets say that's not the case and you have a need to process the IDoc errors individually, you can change the standard task to your own (new) task and in the IDoc processing raise an event (I think there might be customizing/fuction module you can call in the IDoc processing WE20 transaction) to end the task when the IDoc is processed successfully.