give user notification if bot or task not found

appengine/swarming/ui/res/imp/common/common-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.
 
   It contains the definition of the following Behaviors:
 
     SwarmingBehaviors.CommonBehavior
 
   To use it, include
@@ skipping 59 matching lines @@
           console.log("Need at least a polymer element to bind to, a busy element, and a url");
           return;
         }
         this.set(busy, true);
         var now = new Date();
         this._jsonAsync = this._jsonAsync || {};
         this._jsonAsync[bindTo] = now;
         if (params) {
           url = url + "?" + sk.query.fromParamSet(params);
         }
-        sk.request("GET", url, "", headers).then(JSON.parse).then(function(json){
+        return sk.request("GET", url, "", headers).then(JSON.parse).then(function(json){
           if (this._jsonAsync[bindTo] !== now) {
             console.log("ignoring result because a second request happened.");
             this.set(busy, false);
             return;
           }
           this.set(bindTo, json);
           this.set(busy, false);
         }.bind(this)).catch(function(reason){
           console.log("Reason for failure of request to " + url, reason);
 
           if (this._jsonAsync[bindTo] !== now) {
             console.log("ignoring failure because a second request happened.");
             this.set(busy, false);
             return;
           }
           this.set(bindTo, false);
           this.set(busy, false);
+          return Promise.reject(reason);

[nodir, 2016/11/02 18:46:29]

What is reason? Can a caller check HTTP status code?
i think the _getJsonAsync's comment needs to be updated.

[kjlubick, 2016/11/03 15:48:35]

reason is (after https://skia-review.googlesource.com/c/4372/ lands) an 
{
status: Number,
reason: String, # error message from the server
}

[nodir, 2016/11/03 18:34:16]

i think it should a part of the comment. The fact that this function uses sk is
an implementation detail and thus it is not obvious that it will have the same
format

[kjlubick, 2016/11/03 19:23:54]

Done.

         }.bind(this));
       },
 
       _humanDuration: function(timeInSecs) {
         return sk.human.strDuration(timeInSecs) || "0s";
       },
 
       _not: function(a) {
         return !a;
       },
@@ skipping 43 matching lines @@
         }
         return this._humanDuration((second.getTime() - first.getTime())/1000);
       },
 
       _truthy: function(a){
         return !!a;
       }
     };
   })();
 </script>