More Robust Handling of Missing Futures Expiries

[dougransom]

Due to incorrect information about final trade of of vix futures, or any other reason, some decent feedback should be provided to caller/user instead of just a bunch of missing data. So far this happens mainly if the Pandas Market Calendar either isn't the latest version, or doesn't have a holiday correctly recorded for CFE. But it could also happen if the futures data were not available for a particular expiry for some other reason. As an example #31 a future wasn't downloaded due to an error in vix_utils itself, and manifested by confusing the caller with silently missing data.