Milo: Add step text for swarming task exceptions

appengine/cmd/milo/swarming/testdata/build-canceled.swarm

 {
  "created_ts": "2016-03-14T10:00:01.468360",
  "bot_version": "02233557e20b68d33d7812c0465b582ed288a9bb",
  "task_id": "2d8d62f6be545b10",
  "tags": [
   "buildbucket_bucket:estaab-infra-waterfall-test",
   "buildbucket_hostname:cr-buildbucket-dev.appspot.com",
   "builder:infra-continuous-precise-32",
   "cores:8",
   "cron_invocation_id:9183652815541395264",
   "cron_job_id:infra-experimental/buildbucket-job-estaab-infra-waterfall-test",
   "master:master.estaab-infra-waterfall-test",
   "os:Ubuntu-12.04",
   "pool:default",
   "priority:100",
   "recipe_name:infra/infra_continuous",
   "recipe_repository:https://chromium.googlesource.com/chromium/tools/build",
   "recipe_revision:",
   "user:",
   "user_agent:luci-cron",
   "allow_milo:1"
  ],
  "internal_failure": false,
  "costs_usd": [
   0.5220010006994543
  ],
  "bot_dimensions": [
   {
-   "value": [
+  "value": [
     "8"
    ],
    "key": "cores"
   },
   {
    "value": [
     "x86",
     "x86-64"
    ],
    "key": "cpu"
@@ skipping 38 matching lines @@
    ],
    "key": "zone"
   }
  ],
  "server_versions": [
   "1931-8ffe4d1"
  ],
  "exit_code": "-15",
  "name": "buildbucket-estaab-infra-waterfall-test-infra-continuous-precise-32",
  "failure": true,
- "state": "TIMED_OUT",
+ "state": "CANCELED",
  "modified_ts": "2016-03-14T11:00:18.486020",
  "user": "",
  "bot_id": "swarm3-c4",
  "duration": 3615.243180990219,
  "try_number": "1",
  "completed_ts": "2016-03-14T11:00:18.486020",
  "started_ts": "2016-03-14T10:00:01.850710",
  "kind": "swarming#taskItem",
  "etag": "\"8T7hve1eF1AtJxly4TygH4Vsei8/BOiXFFzC_twa1KB8vfr-umqxc4Q\""
 }