Milo: Add step text for swarming task exceptions

appengine/cmd/milo/swarming/expectations/build-canceled.json

 {
   "Summary": {
     "Label": "",
-    "Status": "InfraFailure",
+    "Status": "Failure",
     "Bot": {
       "Label": "swarm3-c4",
       "Img": "",
       "URL": "https://debug/restricted/bot/swarm3-c4",
       "Alt": ""
     },
     "Source": {
       "Label": "Task 2d8d62f6be545b10",
       "Img": "",
       "URL": "https://debug/user/task/2d8d62f6be545b10",
       "Alt": ""
     },
     "MainLink": null,
     "SubLink": null,
     "Progress": null,
     "Started": "2016-03-14T10:00:01.85071Z",
     "Finished": "2016-03-14T10:00:01.85071Z",
     "Duration": 3615243180990,
     "Type": "Recipe",
     "LevelsDeep": 0,
-    "Text": null
+    "Text": [
+      "Task cancelled by user"
+    ]
   },
   "SourceStamp": null,
   "Components": [
     {
       "Label": "recipe bootstrap",
       "Status": "Success",
       "Bot": null,
       "Source": null,
       "MainLink": {
         "Label": "stdout",
@@ skipping 40 matching lines @@
       ]
     }
   ],
   "PropertyGroup": [
     {
       "GroupName": "Swarming",
       "Property": [
         {
           "Key": "Cost of job (USD)",
           "Value": "$0.52"
-        },
-        {
-          "Key": "Exit Code",
-          "Value": "-15"
         }
       ]
     },
     {
       "GroupName": "Swarming Tags",
       "Property": [
         {
           "Key": "buildbucket_bucket",
           "Value": "estaab-infra-waterfall-test"
         },
@@ skipping 84 matching lines @@
       "GroupName": "setup_build",
       "Property": null
     },
     {
       "GroupName": "Main",
       "Property": null
     }
   ],
   "Blame": null
 }