Why did completeTasks error?

I am creating a webApi to trigger a!completeTask().  If this encounters an error, I return an httpResponse to indicate such.

The question is:  how do I know what the error was?  Is there some way to return the task completion error in the webApi?

Thank you.

  a!completeTask(
    taskId: ri!taskId,
    taskInputs: {
      someData: "abc",
      moreData: "def"
    },
    onSuccess: a!httpResponse(200, {}, "Success")
    onError: a!httpResponse(500, {}, "What was the error??")
  )