google: add utilities supporting upcoming oauth2 functionality

These are used to support some extended utilities to help with STS requests.

Change-Id: Iafc145b06ca42374cfc2ac6572762a50bcf560f2
GitHub-Last-Rev: 3085fe5703
GitHub-Pull-Request: golang/oauth2#439
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/259777
Trust: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

google/internal/externalaccount/err.go

@@ -0,0 +1,18 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import "fmt"
+
+// Error for handling OAuth related error responses as stated in rfc6749#5.2.
+type Error struct {
+	Code        string
+	URI         string
+	Description string
+}
+
+func (err *Error) Error() string {
+	return fmt.Sprintf("got error code %s from %s: %s", err.Code, err.URI, err.Description)
+}