Fix traceback handling for fake tasks

[bjester]

Summary

Description of the change(s) you made

• I've looked into this issue and I confirm that it is reproducible, but currently it's hard to see what's going wrong
• This fixes an issue in capturing the error and traceback of fake tasks like copying so I can better debug the referenced issue

References

#3526

Comments

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[LianaHarris360]

I see the switch to format_exc() will return a string and allow for an easier debugging experience. It looks good to me!