Light client doesn't properly fail if node doesn't support `eth_getProof` endpoint

Using Rigil's default RPC node for light client, which does not provide `eth_getProof` endpoint and returns 403, the following was the result of requesting execution within the kettle:

```
{"Success":{"reason":"Stop","gas_used":21064,"gas_refunded":0,"logs":[],"output":{"Call":"0x"}}}
```

We would prefer a clear indication of the problem