Add Device errors to bot-list status

appengine/swarming/ui/res/imp/botlist/bot-list-shared-behavior.html

 <!--
   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.
 
   window.SwarmingBehaviors.BotListBehavior contains any shared functions and
   constants used by the bot-list and its sub-elements.
 
   To use it, include
       behaviors: [SwarmingBehaviors.BotListBehavior]
@@ skipping 21 matching lines @@
 
       // _attribute looks first in dimension and then in state for the
       // specified attribute. This will always return an array. If there is
       // no matching attribute, ["unknown"] will be returned.
       _attribute: function(bot, attr, none) {
         none = none || UNKNOWN;
         return this._dimension(bot, attr) || this._state(bot, attr) || [none];
       },
 
       _devices: function(bot) {
-        return bot.state.devices;
+        return bot.state.devices || [];
       },
 
       // _deviceType returns the codename of a given Android device.
       _deviceType: function(device) {
         return device.device_type.toLowerCase();
       },
 
       // _dimension returns the given dimension of a bot. If it is defined, it
       // is an array of strings.
       _dimension: function(bot, dim) {
@@ skipping 25 matching lines @@
       _taskId: function(bot) {
         if (bot && bot.task_id) {
           return bot.task_id;
         }
         return "idle";
       },
 
     }];
   })()
 </script>