give user notification if bot or task not found

appengine/swarming/ui/res/imp/botpage/bot-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:
 
     <bot-page>
 
@@ skipping 112 matching lines @@
       <h2 hidden$="[[_signed_in]]">You must sign in to see anything useful.</h2>
 
       <div hidden$="[[_not(_signed_in)]]">
 
         <bot-page-data
           id="data"
           auth_headers="[[_auth_headers]]"
           bot_id="[[bot_id]]"
 
           bot="{{_bot}}"
+          bot_exists="{{_bot_exists}}"
           busy="{{_busy1}}"
           events="{{_events}}"
           tasks="{{_tasks}}"
           on-reload="_clearAndReload">
         </bot-page-data>
 
         <div class="header horizontal layout">
           <paper-input class="id_input" label="Bot id" value="{{bot_id}}"></paper-input>
           <template is="dom-if" if="[[_ccLink(_bot)]]">
             <div class="vertical layout">
               <a href$="[[_ccLink(_bot)]]" class="cloud">Cloud Console</a>
             </div>
           </template>
           <button on-click="_refresh">
             <iron-icon class="refresh" icon="icons:refresh"></iron-icon>
           </button>
         </div>
+      </div>
 
+      <h2 hidden$="[[_bot_exists]]">Bot not found.</h2>
+
+      <div hidden$="[[_not(_bot_exists)]]">
         <div class="horizontal wrap layout">
           <div class="flex">
             <table>
               <tr class$="[[_isDead(_bot)]]" title="Last time the bot contacted the server.">
                 <td>Last Seen</td>
                 <td title="[[_bot.human_last_seen_ts]]">
                   [[_timeDiffExact(_bot.last_seen_ts)]] ago</td>
                 <td>
                 <!-- dom-ifs are slightly less performant than hidden$=, but
                 prevent things from first drawing and then hiding. We prefer to
@@ skipping 443 matching lines @@
       },
 
       _toggleState: function() {
         this.set("_show_state", !this._show_state);
       }
 
     });
   })();
   </script>
 </dom-module>