[errors] Greatly simplify common/errors package.

logdog/server/archivist/archivist.go

 // Copyright 2016 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 archivist
 
 import (
         "bytes"
         "encoding/hex"
         "fmt"
@@ skipping 520 matching lines @@
                 Project:  sa.project,
                 Path:     sa.path,
                 KeysOnly: true,
         }
 
         nextIndex := types.MessageIndex(0)
         var ierr error
         err := sa.Storage.Get(sreq, func(e *storage.Entry) bool {
                 idx, err := e.GetStreamIndex()
                 if err != nil {
-                        ierr = errors.Annotate(err).Reason("could not get stream index").Err()
+                        ierr = errors.Annotate(err, "could not get stream index").Err()
                         return false
                 }
 
                 switch {
                 case idx != nextIndex:
                         ierr = fmt.Errorf("missing log entry index %d (next %d)", nextIndex, idx)
                         return false
 
                 case idx == sa.terminalIndex:
                         // We have hit our terminal index, so all of the log data is here!
@@ skipping 248 matching lines @@
         return e.inner
 }
 
 func isFailure(err error) bool {
         if err == nil {
                 return false
         }
         _, ok := err.(*statusErrorWrapper)
         return !ok
 }