fix: improve handling response from fullnode's /submit_block

[luislhl]

Summary

The /submit_block endpoint in hathor-core can return 200 for errors, with the errors in the response body.

tx-mining-service is relying only in the response status-code to check if something went wrong.

We should make it read the response body when sending requests to /submit_block.

Specifically, we should make sure we treat the case where hathor.manager.HathorManager.submit_block method in hathor-core can return False in the response.