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