Redirects to an invalid page are not handled

[robrwo]

This is similar to #3:

1. You visit a bookkmarked URL
2. The server redirects you to a second URL
3. The add-on alerts you that the bookmark has redirected, but...
4. The URL is invalid.

When it has a redirection (or even a recommended change for canonical urls), it should attempt to request the URL to check if it is valid. If the redirection is invalid, then it should instead treat it as inacessible.

(If a canonical URL is invalid, then it should just ignore it.)