Consider rolling back trust after verification if signature upload fails

If the signature upload fails, we'll actually send a `.cancel` to the other party, so they won't mark us as verified, but we have already marked them as verified locally. We'll never retry uploading their signature though. So things are left in a bit of an inconsistent state, where you can't retry verification from the failing side as the user is marked as verified. The fact that you can't unverify currently makes this more annoying. see https://github.com/vector-im/riot-web/issues/13514#issuecomment-624551559