give user notification if bot or task not found

appengine/swarming/ui/res/imp/taskpage/task-page.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.
 
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <task-page>
 
@@ skipping 106 matching lines @@
       <div hidden$="[[_not(_signed_in)]]">
 
         <task-page-data
           id="data"
           auth_headers="[[_auth_headers]]"
           task_id="[[task_id]]"
 
           busy="{{_busy}}"
           request="{{_request}}"
           result="{{_result}}"
-          stdout="{{_stdout}}">
+          stdout="{{_stdout}}"
+          task_exists="{{_task_exists}}">
         </task-page-data>
 
         <div class="horizontal layout wrap">
           <div class="left flex">
             <div class="horizontal layout">
               <paper-input class="id_input" label="Task id" value="{{task_id}}"></paper-input>
               <button on-click="_refresh">
                 <iron-icon class="refresh" icon="icons:refresh"></iron-icon>
               </button>
               <button on-click="_promptRetry">Retry</button>
               <template is="dom-if" if="[[_canCancelTask(_result,_permissions)]]">
                 <button on-click="_promptCancel">Cancel</button>
               </template>
             </div>
-            <table>
+            <h2 hidden$="[[_task_exists]]">Task not found.</h2>
+            <table hidden$="[[_not(_task_exists)]]">
               <tr>
                 <td>Name</td>
                 <td>[[_request.name]]</td>
               </tr>
               <tr>
                 <td>State</td>
                 <td class$="[[_stateClass(_result)]]">[[_state(_result)]]</td>
               </tr>
               <tr>
                 <td>Created</td>
@@ skipping 185 matching lines @@
                 </tr>
                 <template is="dom-repeat" items="[[_request.properties.caches]]" as="cache">
                   <tr>
                     <td><span>[[cache.name]]</span>:<span>[[cache.path]]</span></td>
                   </tr>
                 </template>
 
               </template>
             </table>
 
-            <div class="title">Task Execution</div>
+            <div class="title" hidden$="[[_not(_task_exists)]]">Task Execution</div>
             <template is="dom-if" if="[[_wasPickedUp(_result)]]">
-              <table>
+              <table hidden$="[[_not(_task_exists)]]">
                 <tr>
                   <td>Bot assigned to task</td>
                   <td><a href$="[[_botLink(_result.bot_id)]]">[[_result.bot_id]]</td>
                 </tr>
                 <tr>
                   <td rowspan$="[[_rowspan(_result.bot_dimensions)]]">
                     <a>Bot Dimensions</a>
                   </td>
                 </tr>
                 <template is="dom-repeat" items="[[_result.bot_dimensions]]" as="dimension">
@@ skipping 57 matching lines @@
                 </tr>
                 <tr>
                   <td>Initial bot cache</td>
                   <td>[[_result.performance_stats.isolated_download.initial_number_items]] items;
                   [[_bytes(_result.performance_stats.isolated_download.initial_size)]]</td>
                 </tr>
               </table>
             </template>
           </div>
 
-          <div class="flex right">
+          <div class="flex right" hidden$="[[_not(_task_exists)]]">
             <div class="horizontal layout">
               <div class="tabs">
                 <paper-tabs selected="{{_show_raw}}" no-bar>
                   <paper-tab disabled$="[[_noMilo(_request)]]">Milo Output</paper-tab>
                   <paper-tab>Raw Output</paper-tab>
                 </paper-tabs>
               </div>
 
               <paper-input
                 class="refresh_input"
@@ skipping 316 matching lines @@
         return result && result.state !== this.PENDING && result.state !== this.CANCELED && result.state != this.EXPIRED;
       },
 
       _wasNotPickedUp: function(result) {
         return result && !this._wasPickedUp(result);
       },
     });
   })();
   </script>
 </dom-module>