Handle most of the 404 errors

In Angular it's recommended to use `<img ng-src="...` rather than `<img
src="...`, source: http://docs.angularjs.org/api/ng.directive:ngSrc

However, there is still a `GET
http://127.0.0.1:8000/static/products/img/ 404 (NOT FOUND)` error when
displaying a single product, for example
http://127.0.0.1:8000/#/products/2

Issue #3