Add more type checking to transactions queued by Herald

Summary:
See PHI1096. Depends on D20213. An install is reporting a hard-to-reproduce issue where a non-transaction gets queued by Herald somehow. This might be in third-party code.

Sprinkle the relevant parts of the code with final and type checking to try to catch the problem before it causes a fatal we can't pull a stack trace out of.

Test Plan: Poked around locally (e.g., edited revisions to cause Herald to trigger), but hard to know if this will do what it's supposed to or not without deploying and seeing if it catches anything.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20214