Fails to build Operations Table if one of the backends is missing

[tokebe]

Ideally, Retriever should tolerate missing backends (related: #47), but especially it should handle something basic like building an operations table from whichever backend(s) are available.

[tokebe]

I'd also like to build a simple /status endpoint which gives the connection status of all drivers (Tier 1, Tier 0, Redis, Mongo), the operation table size, etc.