在directive中定义组件，在ng-src错误时，调用err-src.
app.directive('errSrc',function() {
return {
link: function(scope, element, attrs) {
if(attrs.src='undefined'){
attrs.$set('src', attrs.errSrc);
}
element.bind('error', function() {
if (attrs.src != attrs.errSrc) {
attrs.$set('src', attrs.errSrc);
}
});
}
}
});