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..689eff7b3dc8a17c706751b92943ccc0da6413ea 100644
--- a/appengine/swarming/ui/res/imp/common/common-behavior.html
+++ b/appengine/swarming/ui/res/imp/common/common-behavior.html
@@ -77,7 +77,7 @@
         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 +95,7 @@
           }
           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));
       },