Sage should gracefully handle fields that no longer have data (consider saving DV configuration).

[kaladay]

Is your feature request related to a problem? Please describe.
This is the result of the spike investigation of #500.

Describe the solution you'd like
The Discovery View should remain editable when the network connection fails.
The Discovery View should remain functional on schema changes where data no longer exists.
Consider adding warnings or alerts (but not on the public view, instead for a manager).

Additional context

Consider the situation:

1. A new core is indexed.
2. A discovery view is configurade based on that now indexed core.
3. The core is deleted and the then indexed again.
4. The discovery view may have fields selected for use that no longer exist in the discovery view.
5. Errors happen, mostly on the backend and the results might just be empty.
6. Editing the discovery view may cause the page fields to be empty and prevent proper saving.

This problem also happens on network errors and the data is not available.