Use `futures` to parallelize calls to `_send_request_to_node()`

This allows queries that need to go to multiple brokers to be run in parallel.

`getattr(object, 'x', object.y)` will evaluate the default argument
`object.y` regardless of whether `'x'` exists.

For details see: https://stackoverflow.com/q/31443989/770425

Previously we weren't accounting for when the response tuple also has a
`error_message` value.

Note that in Java, the error fieldname is inconsistent:
 - `CreateTopicsResponse` / `CreatePartitionsResponse` uses `topic_errors`
 - `DeleteTopicsResponse` uses `topic_error_codes`
So this updates the `CreateTopicsResponse` classes to match.

The fix is a little brittle, but should suffice for now.

`isinstance()` won't work here, as the types require identity comparison.