Add proper error handling for status codes
Add proper error handling for status codes

> The Promise returned from fetch() won't reject on HTTP error status 
even if the response is a HTTP 404 or 500. Instead, it will resolve 
normally, and it will only reject on network failure, or if anything 
prevented the request from completing.

See https://github.com/github/fetch#handling-http-error-statuses
ello/webapp,ello/webapp,ello/webapp