rpc: add HTTPError type for HTTP error responses (#22677)

The new error type is returned by client operations contains details of
the response error code and response body.

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/http.go

@@ -134,19 +134,11 @@ func DialHTTP(endpoint string) (*Client, error) {
 func (c *Client) sendHTTP(ctx context.Context, op *requestOp, msg interface{}) error {
 	hc := c.writeConn.(*httpConn)
 	respBody, err := hc.doRequest(ctx, msg)
-	if respBody != nil {
-		defer respBody.Close()
-	}
-
 	if err != nil {
-		if respBody != nil {
-			buf := new(bytes.Buffer)
-			if _, err2 := buf.ReadFrom(respBody); err2 == nil {
-				return fmt.Errorf("%v: %v", err, buf.String())
-			}
-		}
 		return err
 	}
+	defer respBody.Close()
+
 	var respmsg jsonrpcMessage
 	if err := json.NewDecoder(respBody).Decode(&respmsg); err != nil {
 		return err
@@ -194,7 +186,17 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 		return nil, err
 	}
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return resp.Body, errors.New(resp.Status)
+		var buf bytes.Buffer
+		var body []byte
+		if _, err := buf.ReadFrom(resp.Body); err == nil {
+			body = buf.Bytes()
+		}
+
+		return nil, HTTPError{
+			Status:     resp.Status,
+			StatusCode: resp.StatusCode,
+			Body:       body,
+		}
 	}
 	return resp.Body, nil
 }