Add debugging for no disk

[ribalba]

We are currently seeing that in some cases power metrics does not report a disk being present. We should add some debugging to see when this happens as normally a disk doesn't vanish.

[ArneTR]

I patched this in for the moment which will give me an OS notification:

c.execute('INSERT INTO measurements (time, data, uploaded) VALUES (?, ?, 0)',
                    (data['timestamp'], compressed_data_str))

            subprocess.check_output(['osascript', '-e', 'display notification "Disk is empty" with title "PowerHog Logger Warning" subtitle time string of (current date)'])
            logging.info('Disk is empty!')