Handle non successfull HTTP responses

This change set equips the RPC client with handling of non successful
HTTP responses. An HTTP response is considered non successful when its
status code is not in the range 200..299

infrastructure.go

@@ -614,22 +614,30 @@ func (c *Client) handleSendPostMessage(details *sendPostDetails) {
 	log.Tracef("Sending command [%s] with id %d", cmd.Method(), cmd.Id())
 	httpResponse, err := c.httpClient.Do(details.request)
 	if err != nil {
-		details.responseChan <- &response{result: nil, err: err}
+		details.responseChan <- &response{err: err}
 		return
 	}
 
 	// Read the raw bytes and close the response.
 	respBytes, err := btcjson.GetRaw(httpResponse.Body)
 	if err != nil {
-		details.responseChan <- &response{result: nil, err: err}
+		details.responseChan <- &response{err: err}
 		return
 	}
+
+	// Handle unsuccessful HTTP responses
+	if httpResponse.StatusCode < 200 || httpResponse.StatusCode >= 300 {
+		details.responseChan <- &response{err: errors.New(string(respBytes))}
+		return
+	}
+
 	var resp rawResponse
 	err = json.Unmarshal(respBytes, &resp)
 	if err != nil {
-		details.responseChan <- &response{result: nil, err: err}
+		details.responseChan <- &response{err: err}
 		return
 	}
+
 	res, err := resp.result()
 	details.responseChan <- &response{result: res, err: err}
 }