I am creating a Map/Reduce task in a RESTlet that is working as I hoped - however, does anyone know if there's a way for said Map/Reduce task to return any errors back to the source that scheduled it?

Pass the email of the user as a task parameter you wish to view the errors to the MR script and then in the summarize stage, send the script logs filtered by type = error.

@Eric B Thanks for the recommendation

You can also iterate the errors for map and reduce in the summarise stage which may be a neater set of data since it's specific to "this" run, and email that out in a nice format.