The default behavior is to reject the error. If you don't do that, you are hiding the error condition.

gistfile1.txt

angular.module('httpProgress')
    .factory('httpProgressInterceptor', function($q) {
        var requestCount = 0;

        function updateProgress(update) {
            var previous = requestCount;
            requestCount += update;
            
            if(previous <= 0 && requestCount > 0) {
                NProgress.start();
            } else if (previous > 0 && requestCount <= 0) {
                NProgress.done(true);
            } else {
                NProgress.inc();
            }
        }

        function incrementRequests() {
            updateProgress(1);
        }
        
        function decrementRequests() {
            updateProgress(-1);
        }

        return {
            request: function (request) {
                incrementRequests();
                return request;
            },
            
            response: function (response) {
                decrementRequests();
                return response;
            },
            
            responseError: function (rejection) {
                decrementRequests();
                return $q.reject(rejection);
            }
        };
    })
    .config(function ($httpProvider) {
        $httpProvider.interceptors.push('httpProgressInterceptor');
    });