Fix countdown embeds and endless loops on resource loads in Javelin

Summary:
Fixes T5273. Two issues:

  - If a JX.Resource callback throws, we keep running it. Instead, make sure it gets cleaned up before raising an exception.
  - The countdown timer script doesn't recover gracefully if the node has been removed from the document by the time it runs. Instead, just bail if we can't find the countdown.

Test Plan: Dumped `{Cxyz}` into a preview and got a countdown.

Reviewers: yungsters

Reviewed By: yungsters

Subscribers: epriestley

Maniphest Tasks: T5273

Differential Revision: https://secure.phabricator.com/D9399