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 or something.

Test Plan: Verified tokenizers still work correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5623