Returns identity and transaction information about a confirmed block in the ledger, using the given configuration.
A slot that holds no block is answered by the node with a JSON-RPC error,
not with null: -32007 (slot skipped), -32001 (pruned from the ledger)
or -32004 (not yet produced or rooted). The transport raises such errors
as a JsonRpcException (CallException) that runWeb3' does not catch,
so wrap the call in try to handle them. Nothing is
returned only when the node itself answers null, which Agave does solely
for a rooted slot missing from its blockstore without being flagged skipped.