give user notification if bot or task not found

appengine/swarming/ui/res/imp/common/common-behavior.html

Index: appengine/swarming/ui/res/imp/common/common-behavior.html
diff --git a/appengine/swarming/ui/res/imp/common/common-behavior.html b/appengine/swarming/ui/res/imp/common/common-behavior.html
index 68712d51a2f3dc7b1db7eaa7f64c166f56602a4b..031af1ebea46fdf3f443a47d40ff9c03a9b26d2c 100644
--- a/appengine/swarming/ui/res/imp/common/common-behavior.html
+++ b/appengine/swarming/ui/res/imp/common/common-behavior.html
@@ -59,9 +59,10 @@
       // set to true while the request is in flight and false afterwards.
       // request headers (e.g. authentication) and query params will be used if
       // provided. Query params is an object like {String:Array<String>}. On
-      // error, bindTo will be set to false.  It is not set to undefined
-      // because computed values in Polymer don't fire if a property is
-      // undefined. Clients should check that bindTo is not falsey.
+      // error, bindTo will be set to false and the promise will be rejected.
+      // It is not set to undefined because computed values in Polymer don't
+      // fire if a property is undefined.
+      // Clients should check that bindTo is not falsey.
       // To avoid multiple requests clobering one another, an object _jsonAsync
       // is created on "this" to debounce requests - the most recent request
       // will win out.
@@ -77,7 +78,7 @@
         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);
@@ -95,6 +96,7 @@
           }
           this.set(bindTo, false);
           this.set(busy, false);
+          return Promise.reject(reason);
         }.bind(this));
       },