add audit/admin trace support for browser requests (#10947)

To support this functionality we had to fork
the gorilla/rpc package with relevant changes

pkg/rpc/json2/error.go

@@ -0,0 +1,44 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copyright 2020 MinIO, Inc. All rights reserved.
+// forked from https://github.com/gorilla/rpc/v2
+// modified to be used with MinIO under Apache
+// 2.0 license that can be found in the LICENSE file.
+
+package json2
+
+import (
+	"errors"
+)
+
+type ErrorCode int
+
+const (
+	E_PARSE       ErrorCode = -32700
+	E_INVALID_REQ ErrorCode = -32600
+	E_NO_METHOD   ErrorCode = -32601
+	E_BAD_PARAMS  ErrorCode = -32602
+	E_INTERNAL    ErrorCode = -32603
+	E_SERVER      ErrorCode = -32000
+)
+
+var ErrNullResult = errors.New("result is null")
+
+type Error struct {
+	// A Number that indicates the error type that occurred.
+	Code ErrorCode `json:"code"` /* required */
+
+	// A String providing a short description of the error.
+	// The message SHOULD be limited to a concise single sentence.
+	Message string `json:"message"` /* required */
+
+	// A Primitive or Structured value that contains additional information about the error.
+	Data interface{} `json:"data"` /* optional */
+}
+
+func (e *Error) Error() string {
+	return e.Message
+}