[errors] Greatly simplify common/errors package.

milo/build_source/swarming/build.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 swarming
 
 import (
         "bytes"
         "fmt"
         "net/http"
@@ skipping 488 matching lines @@
 
 func (bl *buildLoader) newEmptyAnnotationStream(c context.Context, addr *types.StreamAddr) (
         *raw_presentation.AnnotationStream, error) {
 
         fn := bl.logDogClientFunc
         if fn == nil {
                 fn = raw_presentation.NewClient
         }
         client, err := fn(c, addr.Host)
         if err != nil {
-                return nil, errors.Annotate(err).Reason("failed to create LogDog client").Err()
+                return nil, errors.Annotate(err, "failed to create LogDog client").Err()
         }
 
         as := raw_presentation.AnnotationStream{
                 Client:  client,
                 Project: addr.Project,
                 Path:    addr.Path,
         }
         if err := as.Normalize(); err != nil {
-                return nil, errors.Annotate(err).Reason("failed to normalize annotation stream parameters").Err()
+                return nil, errors.Annotate(err, "failed to normalize annotation stream parameters").Err()
         }
 
         return &as, nil
 }
 
 // failedToStart is called in the case where logdog-only mode is on but the
 // stream doesn't exist and the swarming job is complete.  It modifies the build
 // to add information that would've otherwise been in the annotation stream.
 func failedToStart(c context.Context, build *resp.MiloBuild, res *swarming.SwarmingRpcsTaskResult, host string) error {
         var err error
@@ skipping 66 matching lines @@
         // If the Swarming task explicitly specifies its log location, we prefer that.
         // As a fallback, we will try and parse the Swarming task's output for
         // annotations.
         switch {
         case logDogStreamAddr != nil:
                 logging.Infof(c, "Loading build from LogDog stream at: %s", logDogStreamAddr)
 
                 // If the LogDog stream is available, load the step from that.
                 as, err := bl.newEmptyAnnotationStream(c, logDogStreamAddr)
                 if err != nil {
-                        return nil, errors.Annotate(err).Reason("failed to create LogDog annotation stream").Err()
+                        return nil, errors.Annotate(err, "failed to create LogDog annotation stream").Err()
                 }
 
                 prefix, _ := logDogStreamAddr.Path.Split()
                 ub = &raw_presentation.ViewerURLBuilder{
                         Host:    logDogStreamAddr.Host,
                         Prefix:  prefix,
                         Project: logDogStreamAddr.Project,
                 }
 
                 if s, err = as.Fetch(c); err != nil {
@@ skipping 152 matching lines @@
         for _, tag := range v {
                 var value string
                 parts := strings.SplitN(tag, ":", 2)
                 if len(parts) == 2 {
                         value = parts[1]
                 }
                 res[parts[0]] = value
         }
         return res
 }