This tweaks how Dstat data is loaded to handle errors more cleanly.
Attempts to load dstat data for datasets that don't have it configured
will fail early and avoid making a request, and 404 errors are now
handled appropriately. Also, the Dstat parser now validates
headers to make sure it is actually parsing Dstat CSV data. Lastly,
the chart is now hidden until data is loaded successfully, removing
the large blank area previously shown for missing datasets.
Change-Id: I96b8d038af1cc718ae9c43a078679a04533fddc5
d42aed0935