Don't set a link that errors as pending

[lunakv]

Sometimes WotC messes up their update process, and things happen. For example, a new CR is released but the TXT link returns 404.

To avoid having to manually check/reset the pending links, we should make a basic check that the parsed link at least returns an actual response before saving it.

[lunakv]

Encountering this condition (an erroring link) should probably still create a notification though? There's also a discussion to be had about how to handle the situation when a link that's already pending changes again.