give user notification if bot or task not found

appengine/swarming/ui/res/imp/botpage/bot-page-data.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-data>
 
@@ skipping 79 matching lines @@
         busy: {
           type: Boolean,
           computed: "_or(_busy1)",
           notify: true,
         },
         bot: {
           type: Object,
           computed: "_parseBot(_bot)",
           notify: true,
         },
+        bot_exists: {
+          type: Boolean,
+          value: true,
+          notify: true,
+        },
 
         // private
          _busy1: {
           type: Boolean,
           value: false
         },
         _bot: {
           type: Object,
         },
         _events: {
@@ skipping 12 matching lines @@
         if (!this.bot_id || !this.auth_headers) {
           return;
         }
         if (lastRequest) {
           this.cancelAsync(lastRequest);
         }
 
         lastRequest = this.async(function(){
           lastRequest = undefined;
           var baseUrl = "/api/swarming/v1/bot/"+this.bot_id;
-          this._getJsonAsync("_bot", baseUrl + "/get",
+          var p = this._getJsonAsync("_bot", baseUrl + "/get",
             "_busy1", this.auth_headers);
+          p.then(function(){
+            this.set("bot_exists", true);
+          }.bind(this)).catch(function(r){
+            if (r.status === 404) {
+              this.set("bot_exists", false);
+            } else {
+              sk.errorMessage("Http response: " + (r.status || " ") + " " + r.response);
+            }
+          }.bind(this));
           this.fire("reload", {id: this.bot_id});
         }, BOT_ID_DEBOUNCE_MS);
 
       },
 
       _parseBot: function(bot) {
         if (!bot) {
           return {};
         }
         // Do any preprocessing here
@@ skipping 94 matching lines @@
           return b.started_ts - a.started_ts;
         });
 
         return tasks;
       }
 
     });
   })();
   </script>
 </dom-module>