Added helpful errors if the stream has stopped

    def move_data(self, istart, istop=None):

        "Extract sharpness as a function of (interpolated) z"

        global np, logging

        if istop is None:

            istop = istart + 2

        jpeg_times = np.array(self.jpeg_times)

        jpeg_sizes = np.array(self.jpeg_sizes)

        stage_times = np.array(self.stage_times)[istart:istop]

        stage_zs = np.array(self.stage_positions)[istart:istop, 2]

        try:

            start = np.argmax(jpeg_times > stage_times[0])

            stop = np.argmax(jpeg_times > stage_times[1])

        except ValueError as e:

            if np.sum(jpeg_times > stage_times[0]) == 0:

                raise ValueError("No images were captured during the move of the stage.  Perhaps the camera is not streaming images?")

            else:

                raise e

        if stop < 1:

            stop = len(jpeg_times)

            logging.debug("changing stop to {}".format(stop))

    def sharpest_z_on_move(self, index):

        """Return the z position of the sharpest image on a given move"""

        jt, jz, js = self.move_data(index)

        if len(js) == 0:

            raise ValueError("No images were captured during the move of the stage.  Perhaps the camera is not streaming images?")

        return jz[np.argmax(js)]

    def data_dict(self):