R1:a7f3316aa3df

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…