return 500 status codes properly when a Stacks block could not be ingested #646

The HTTP server that consumes data from the Stacks node always returns a 200 status code regardless if the block has been ingested correctly or not, because the response handlers never actually return an error, the error is only reflected in the body:

return Json(json!({
    "status": 500,
    "result": format!("Unable to standardize stacks block {}", e),
}));

Because of this, the Stacks node continues to advance the chain creating gaps in the index that are impossible to recover from. This is related to #623