Recover from error loading forum thread list

When a user attempts to load more threads in the forum navigation
sidebar, reset the state of the world so the user can retry, and alert
the user appropriately.

CHANGELOG.rst

@@ -5,6 +5,8 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+LMS: Add error recovery when a user loads more threads in the forum sidebar.
+
 LMS: Add a user-visible alert modal when a forums AJAX request fails.
 
 Blades: Add template for checkboxes response to studio. BLD-193.

common/static/coffee/src/discussion/discussion.coffee

@@ -25,9 +25,8 @@ if Backbone?
         @add model
         model
 
-    retrieveAnotherPage: (mode, options={}, sort_options={})->
-      @current_page += 1
-      data = { page: @current_page }
+    retrieveAnotherPage: (mode, options={}, sort_options={}, error=null)->
+      data = { page: @current_page + 1 }
       switch mode
         when 'search'
           url = DiscussionUtil.urlFor 'search'
@@ -59,6 +58,7 @@ if Backbone?
           @reset new_collection
           @pages = response.num_pages
           @current_page = response.page
+        error: error
 
     sortByDate: (thread) ->
         #   

common/static/coffee/src/discussion/views/discussion_thread_list_view.coffee

@@ -156,7 +156,11 @@ if Backbone?
           $(".post-list a").first()?.focus()
         )
 
-      @collection.retrieveAnotherPage(@mode, options, {sort_key: @sortBy})
+      error = =>
+        @renderThreads()
+        DiscussionUtil.discussionAlert("Sorry", "We had some trouble loading more threads. Please try again.")
+
+      @collection.retrieveAnotherPage(@mode, options, {sort_key: @sortBy}, error)
 
     renderThread: (thread) =>
       content = $(_.template($("#thread-list-item-template").html())(thread.toJSON()))