Return 404s when database contains gaps

[michaelsproul]

Currently blockprint's API will return 0 values when querying a section of the database for which history is missing. This can occur if the beacon node backing blockprint has been falling in and out of sync or is re-syncing and lacks a portion of history.

At the moment these gaps can be detected via the /sync/gaps API, but this is error-prone and not obvious. It would be better if blockprint detected queries affected by the gaps and failed them explicitly so that the user doesn't have to check /sync/gaps manually. E.g. if there is a gap from slot 3623682 to 3687456 (as there is currently) then blockprint should 404 on queries like /blocks_per_client/113625/113850 rather than returning a response with 0 blocks per client for all clients.

The reason the API is the way it is currently is that the SQL query to detect the gaps takes several seconds to evaluate, because it scans the entire table. If this query can be made to run quickly, then it will be feasible to run it for every query.