* Code refactor with reducer and new action type SET_ERROR.
* Add default state for errors and code for SET_ERROR for blogPosts.
* Don't use handleFetchError, instead save the error and blogPosts category into redux store.
* Display message for error and empty content.
* Fix grammar; fix relevant test.
* Code refactor: put action type strings in one place.
* Add tests for error UI.
* Remove illustration for empty content with no blog posts.
Gan Eng Chin