code: func (c *requestContext) err(e error, msg string, args ...interface{}) {
  	code := 0
  	switch {
  	case e == nil:
  		panic("nil")
  	case tq.Retry.In(e):
  		code = 409
  	case transient.Tag.In(e):
  		code = 500
  	default:
  		code = 202 // fatal error, don't need a redelivery
  	}
  	args = append(args, e)
  	c.fail(code, msg+" - %s", args...)
  }
docs: // err sets status to 409 on tq.Retry errors, 500 on transient errors and 202 on
  // fatal ones. Returning status code in range [200–299] is the only way to tell
  // PubSub to stop redelivering the task.