Better error message when block index is loading

```
{"result":null,"error":{"code":-28,"message":"Loading block index..."},"id":null}\n
Unable to make RPC connection to HUSH full node at 127.0.0.1:8822 !
Your HUSH full node is not running, or not accessible at that port :(
See https://myhush.org for more information about this privacy coin
```