Rpc: Return error if block does not exist (#11743) (#11748)

* Return error if block does not exist

* Update docs

(cherry picked from commit 747f8d5877)

Co-authored-by: Tyera Eulberg <teulberg@gmail.com>

core/src/rpc.rs

@@ -578,7 +578,7 @@ impl JsonRpcRequestProcessor {
             self.check_slot_cleaned_up(&result, slot)?;
             Ok(result.ok())
         } else {
-            Ok(None)
+            Err(RpcCustomError::BlockNotAvailable { slot }.into())
         }
     }
 
@@ -650,7 +650,7 @@ impl JsonRpcRequestProcessor {
             self.check_slot_cleaned_up(&result, slot)?;
             Ok(result.ok().unwrap_or(None))
         } else {
-            Ok(None)
+            Err(RpcCustomError::BlockNotAvailable { slot }.into())
         }
     }
 
@@ -4496,7 +4496,7 @@ pub mod tests {
             slot
         );
         let res = io.handle_request_sync(&req, meta);
-        let expected = r#"{"jsonrpc":"2.0","result":null,"id":1}"#;
+        let expected = r#"{"jsonrpc":"2.0","error":{"code":-32004,"message":"Block not available for slot 12345"},"id":1}"#;
         let expected: Response =
             serde_json::from_str(&expected).expect("expected response deserialization");
         let result: Response = serde_json::from_str(&res.expect("actual response"))