Is your feature request related to a problem? Please describe.
I recently imported an OPML file with about 400 feeds in it. About half of them are defunct. I love that FreshRSS provides an indication when the feed url can't be reached, but it would take a lot of time and effort to go through and individually remove each bad feed.

Describe the solution you’d like
I would be very helpful if I could get a button next to the Show only feeds with errors button that let's me remove all feeds with errors.

Describe alternatives you’ve considered
If there is some way I can do this in the database I'm open to it. Also, I would consider some type of tool the checks all the links my OPML file and removes broken ones.