add back off for failing recurrent requests

lms/static/js/instructor_dashboard/util.js

@@ -327,6 +327,7 @@
             this.ms = ms;
             this.fn = fn;
             this.intervalID = null;
+            this.failedTries = 0;
         }
 
         intervalManager.prototype.start = function() {
@@ -344,6 +345,15 @@
             return this.intervalID;
         };
 
+        intervalManager.prototype.failed_retry_threshold = 5;
+
+        intervalManager.prototype.backOff = function() {
+            this.failedTries++;
+            if (this.failedTries >= this.failed_retry_threshold) {
+                this.stop();
+            }
+        };
+
         return intervalManager;
     }());
 
@@ -388,6 +398,9 @@
                         ths.$no_tasks_message.append($('<p>').text(gettext('No tasks currently running.')));
                         return ths.$no_tasks_message.show();
                     }
+                },
+                error: function() {
+                    ths.task_poller.backOff();
                 }
             });
         };
@@ -464,6 +477,9 @@
                     } else {
                         return false;
                     }
+                },
+                error: function() {
+                    ths.downloads_poller.backOff();
                 }
             });
         };