Improve coverage of remarkup "<table>" rule and fix some bad method calls

Summary:
See PHI1334. The existing coverage for "<table>" in remarkup is all testing valid inputs, and we have some incorrect method call names when the input isn't valid.

Add coverage, then make it pass.

Test Plan: Added tests and made them pass. Rendered some questionable <table>-like tags in comment previews.

Reviewers: amckinley

Differential Revision: https://secure.phabricator.com/D20644