Error handling for articles

[hyperreality]

Invalid articles currently go to a blank page, rather than the correct error page. The 400 or 404 errors are sent from the server, they're just not dealt with on the client-side. I don't have the time to fix it up and test it now but I think the below is a start, in articles.client.routes.js (or perhaps it should go into the controller although I don't think the controller is ever reached if there's an error). Paging @mleanos

function getArticle($stateParams, ArticlesService) {
  return ArticlesService.get({
    articleId: $stateParams.articleId
  }).$promise.then(function(data) {
    return data;
  }, function(error) {
    console.log(error);
  }) 
}

[mleanos]

We're already returning a 404 from the server when an Article isn't found. So we just need to add a 404 handler here to send the user to a 404 view. So this should only involve the client-side addition of $http interceptors for 400 & 404.

I don't know what happened to this PR: #1052. But it looks like it will solve this issue.

[hyperreality]

Indeed, that appears to do the trick. However, that approach does have a couple of issues.

1. The error message is generic, as opposed to preserving the "Article not found" error message sent from the server.
2. The back button breaks - the previous page just sends you directly back to the error state.

I will look into those issues and attempt to improve the PR.