feat(ngInclude): emit $includeContentError when HTTP request fails

This adds a scope event notification when a template fails to load.

This can have performance implications, and unfortunately cannot at this moment
be terminated with preventDefault(). But it's nice to be notified when problems
occur!

Closes #5803

src/ng/directive/ngInclude.js

@@ -159,6 +159,16 @@
  * @description
  * Emitted every time the ngInclude content is reloaded.
  */
+
+
+/**
+ * @ngdoc event
+ * @name ng.directive:ngInclude#$includeContentError
+ * @eventOf ng.directive:ngInclude
+ * @eventType emit on the scope ngInclude was declared in
+ * @description
+ * Emitted when a template HTTP request yields an erronous response (status < 200 || status > 299)
+ */
 var ngIncludeDirective = ['$http', '$templateCache', '$anchorScroll', '$animate', '$sce',
                   function($http,   $templateCache,   $anchorScroll,   $animate,   $sce) {
   return {
@@ -227,7 +237,10 @@ var ngIncludeDirective = ['$http', '$templateCache', '$anchorScroll', '$animate'
               currentScope.$emit('$includeContentLoaded');
               scope.$eval(onloadExp);
             }).error(function() {
-              if (thisChangeId === changeCounter) cleanupLastIncludeContent();
+              if (thisChangeId === changeCounter) {
+                cleanupLastIncludeContent();
+                scope.$emit('$includeContentError');
+              }
             });
             scope.$emit('$includeContentRequested');
           } else {

test/ng/directive/ngIncludeSpec.js

@@ -143,6 +143,29 @@ describe('ngInclude', function() {
   }));
 
 
+  it('should fire $includeContentError event when content request fails', inject(
+      function($rootScope, $compile, $httpBackend, $templateCache) {
+    var contentLoadedSpy = jasmine.createSpy('content loaded'),
+        contentErrorSpy = jasmine.createSpy('content error');
+
+    $rootScope.$on('$includeContentLoaded', contentLoadedSpy);
+    $rootScope.$on('$includeContentError', contentErrorSpy);
+
+    $httpBackend.expect('GET', 'tpl.html').respond(400, 'nope');
+
+    element = $compile('<div><div ng-include="template"></div></div>')($rootScope);
+
+    $rootScope.$apply(function() {
+      $rootScope.template = 'tpl.html';
+    });
+    $httpBackend.flush();
+
+    expect(contentLoadedSpy).not.toHaveBeenCalled();
+    expect(contentErrorSpy).toHaveBeenCalledOnce();
+    expect(element.children('div').contents().length).toBe(0);
+  }));
+
+
   it('should evaluate onload expression when a partial is loaded', inject(
       putIntoCache('myUrl', 'my partial'),
       function($rootScope, $compile) {