Milo: Add step text for swarming task exceptions

appengine/cmd/milo/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"
         "encoding/json"
         "fmt"
@@ skipping 201 matching lines @@
 
         switch sr.State {
         case TaskRunning:
                 build.Summary.Status = resp.Running
 
         case TaskPending:
                 build.Summary.Status = resp.NotRun
 
         case TaskExpired, TaskTimedOut, TaskBotDied:
                 build.Summary.Status = resp.InfraFailure
+                switch sr.State {
+                case TaskExpired:
+                        build.Summary.Text = append(build.Summary.Text, "Task expired")
+                case TaskTimedOut:
+                        build.Summary.Text = append(build.Summary.Text, "Task timed out")
+                case TaskBotDied:
+                        build.Summary.Text = append(build.Summary.Text, "Bot died")
+                }
 
         case TaskCanceled:
                 // Cancelled build is user action, so it is not an infra failure.
                 build.Summary.Status = resp.Failure
+                build.Summary.Text = append(build.Summary.Text, "Task cancelled by user")
 
         case TaskCompleted:
 
                 switch {
                 case sr.InternalFailure:
                         build.Summary.Status = resp.InfraFailure
                 case sr.Failure:
                         build.Summary.Status = resp.Failure
                 default:
                         build.Summary.Status = resp.Success
@@ skipping 119 matching lines @@
 // Supports server aliases.
 func taskPageURL(swarmingHostname, taskID string) string {
         return fmt.Sprintf("https://%s/user/task/%s", resolveServer(swarmingHostname), taskID)
 }
 
 // botPageURL returns a URL to a human-consumable page of a swarming bot.
 // Supports server aliases.
 func botPageURL(swarmingHostname, botID string) string {
         return fmt.Sprintf("https://%s/restricted/bot/%s", resolveServer(swarmingHostname), botID)
 }