The range-deleter is deleting orphaned documents by scanning an index compatible with the shard key.

When no index on the shard key is found on a shard trying to perform orphans deletion, the range-deleter starts to spam the logs aggressively and indefinitely with the "Unable to find shard key index" error; this results in a relevant increase on the log file size up to some GBs per hour.

Purpose of this ticket is to:

• Throttle the logging of the problem (e.g. by simply introducing a sleep of X seconds in case the shard key index is not found)
• Change the returned error code from InternalError to the more meaningful IndexNotFound code.