code: def start(self):
          
          self._sensor = cv2.VideoCapture(self._device_id)
          if not self._sensor.isOpened():
              raise Exception("Unable to open OpenCVCameraSensor for id {0}".format(self._device_id))
          self.flush()
docs: Starts the OpenCVCameraSensor Stream
          Raises:
              Exception if unable to open stream