Detect invalid CSV export data and improve user messaging

[mcoles]

Several users have reported a problem (1,2) wherein for whatever reason, the gateway.public.host config setting on Tableau Server was altered when the server was upgraded, causing trusted ticket redemptions to result in HTML responses rather than valid CSV exports from vizzes. When that occurs, VizAlerts unhelpfully diagnoses the issue as a regex parsing problem.

It should be pretty straightforward to look for specific HTML tag characters in the first bit of a CSV export to see if the response seems valid or not. Should try and break with a fake HTML-like viz data set though, just to ensure we don't flag a false positive.