WebUI: Handle non-torrent RSS link

[vafada]

Related issue: #24685

Handle case when attempting to add a invalid torrent from a RSS link via WebUI.

A user can add a RSS url that doesn't contain valid torrent links, for example: https://fosstorrents.com/feed/rss.xml

The desktop can handle it properly by prompting an alert:

The WebUI cannot handle the error and will constantly call fetchMetadata api for the given url until the add torrent dialog is closed.

The Add Torrent Dialog will also keep saying Retrieving Metadata... giving the user false hope since it will NEVER resolve since the url is not a valid torrent. At the same time, the backend will continuously download the file, keep on failing and will keep logging this error on the Logs tab:

qBittorrent/src/webui/api/torrentscontroller.cpp

Line 2450 in 52d3a96

LogMsg(tr("Parse torrent failed. URL: \"%1\". Error: \"%2\".").arg(source, loadResult.error()), Log::WARNING);

This PR keeps a cache of invalid URL and would alert the user if the RSS link they are trying to download is invalid. Also pops and alert saying that url is not a valid torrent file