Improve dataloader resiliance

[michaelwood]

• Unlike other parts of grantnav the dataloader currently isn't covered by sentry.io logging
• ES HTTP connections are less deterministic as they can be influenced by resource availability on the server we could add better connection handling such the retry mechanisms as we implemented in other parts of grantnav https://github.com/ThreeSixtyGiving/grantnav/blob/master/grantnav/frontend/search_helpers.py#L43

related https://github.com/ThreeSixtyGiving/grantnav/issues/1011