[errors] de-specialize Transient in favor of Tags.

common/errors/multierror.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package errors
 
 import (
         "fmt"
 )
 
@@ skipping 30 matching lines @@
 // First returns the first non-nil error.
 func (m MultiError) First() error {
         for _, e := range m {
                 if e != nil {
                         return e
                 }
         }
         return nil
 }
 
-// StackContext implements StackContexter.
-func (m MultiError) StackContext() StackContext {
+func (m MultiError) stackContext() stackContext {
         n, _ := m.Summary()
 
-        return StackContext{
+        return stackContext{
                 InternalReason: "MultiError %(non-nil)d/%(total)d: following first non-nil error.",
                 Data: Data{
                         "non-nil": {Value: n},
                         "total":   {Value: len(m)},
                 },
         }
 }
 
 // NewMultiError create new multi error from given errors.
 //
 // Can be used to workaround 'go vet' confusion "composite literal uses unkeyed
 // fields" or if you do not want to remember that MultiError is in fact []error.
 func NewMultiError(errors ...error) MultiError {
         return errors
 }
 
 // SingleError provides a simple way to uwrap a MultiError if you know that it
 // could only ever contain one element.
 //
 // If err is a MultiError, return its first element. Otherwise, return err.
 func SingleError(err error) error {
         if me, ok := err.(MultiError); ok {
                 if len(me) == 0 {
                         return nil
                 }
                 return me[0]
         }
         return err
 }