Handle RPC Error properly

[cubismic]

Currently, if RPC error occurs, the error message just gives
unexpected end of JSON input
Because the error format provided by the rpc server is as follows:

{
  code: -32032,
  message: 'Not Enough Balance',
  data: 'InsufficientBalance { address: Address(~~), balance: ~, cost: ~ }'
}

unexpected end of JSON input comes from line 75.
(body has the error message in byte slice)

codechain-sdk-go/rpc/base.go

Lines 64 to 77 in 6ad3e76

	resp, err := client.httpClient.Do(req)
	if err != nil {
	return err
	}
	defer resp.Body.Close()
	body, readerr := ioutil.ReadAll(resp.Body)
	if readerr != nil {
	return readerr
	}
	var outerResult Result
	if err := json.Unmarshal(body, &outerResult); err != nil {
	return err
	}