Only open with ROME when the content type is supposed to be RSS
When the user submits an URL, current behavior is to always open it with Rome and consider is an RSS feed if and only if it is done successfully. In case of any exception then it is considered as a website.
This is very bad for 404, UnknownHostException and ConnectionException. To be fixed in #1287 (closed). This is also not that good for page that are not meant to be opened by Rome. If the response from the website tells that the content is supposed to be an HTML page, there is no need to open it with Rome. It's simply not a feed.
To sum up, we should first check if a page can be access and what is the returned content type and only open it with Rome if the Mime type is supposed to be handled by Rome.