`get_chain_head` check if `rpc_request` is error before trying to parse.

The code runs:
```py
self.last_block_hash = result["rpc_request"][0]["result"]
```
But the actual result can be:
```json
{
  "rpc_request": [
    {
      "error": {
        "code": "-32999",
        "message": "'RPC rate limit exceeded'"
      },
      "id": "wP878",
      "jsonrpc": "2.0"
    }
  ]
}
```
Which causes an unexpected `KeyError "result"` which can be confusing.