Handle json-rpc failure response as root response object for batched requests

[tadovas]

Situation:
Some public nodes on various chains reply with the following json body for batched requests:

{ 
  "jsonrpc":"2.0",
  "error":{
     "code":0,
     "message":"we can't execute this request"
  },
  "id":null
}

And since batched method expects Vec<Output> as a result, serde_json fails to parse response and returns generic and obfuscated error like failed to deserialize response: invalid type: map, expected a sequence at line 1 column 0

Solution:
Check for json-rpc error object before parsing response as array of outputs and returns appropriate Rpc error. Test to reproduce the issue and verify the fix is added.

[tomusdrw]

lgtm, thanks!