Improve sequencing of various content/header checks in abstract block diffs

Summary:
Ref T13425. Some diff checks currently sequence incorrectly:

• When we're rendering block lists, syntax highlighting isn't relevant.
• The "large change" guard can prevent rendering of otherwise-renderable changes.
• Actual errors in the document engine (like bad JSON in a ".ipynb" file) aren't surfaced properly.

Improve sequencing somewhat to resolve these issues.

Test Plan:

• Viewed a notebook, no longer saw a "highlighting disabled" warning.
• Forced a notebook to fail, got a useful inline error instead of a popup dialog error.
• Forced a notebook to have a large number of differences, got a rendering out of it.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20843