fix: improve handling response from fullnode's `/submit_block`

# 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.