Investigate remote mirror logic for blocked URLs

I was investigating another issue and noticed the remote mirror update logic appears flawed.

In the UpdateRemoteMirrorService we have a comment that blocked URLs result in a hard fail but the logic tries to retry

I believe we should change this to simply

remote_mirror.hard_fail!(_('The remote mirror URL is invalid.'))

Edit: the previous section appears to be intentional from !57392 (comment 540149493) so we should ideally update the comment to clarify.

I also noticed that we're then returning remote_mirror.last_error which in this case should be the error we just specified _('The remove mirror URL is invalid.'). If that's the case it's probably easier to read and understand if we just return that error e.g.

message = _('The remote mirror URL is invalid.')
remote_mirror.hard_fail!(message)
return error(message)

I'm also wondering if we should also update this error to say "The remote mirror URL is blocked" so it's clear to the end user why this failed.