[errors] Greatly simplify common/errors package.

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 34 matching lines @@
                         return e
                 }
         }
         return nil
 }
 
 func (m MultiError) stackContext() stackContext {
         n, _ := m.Summary()
 
         return stackContext{
-                internalReason: "MultiError %(non-nil)d/%(total)d: following first non-nil error.",
-                data: Data{
-                        "non-nil": {Value: n},
-                        "total":   {Value: len(m)},
-                },
+                internalReason: fmt.Sprintf(
+                        "MultiError %d/%d: following first non-nil error.", n, 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
 }