R1:1e2b6faadd3b

Allow tokenizer construction to fail

Summary: See some discussion in D5622. Javelin explicitly prevents you from putting `<script>` tags into `JX.$H()`, which is probably a good idea, so just let this behavior fail less abruptly instead. We currently have no cases where we load something into a cache and then make a decision about whether to put it into the document or not later on; this should hold us over until we do. If and when we do, we can let those endpoints capture behaviors and replay them later…