Add safety code when creating i-tree region/code pairs for display

This fixes a bug which caused the development server to fail during RRP.
Simplify `_commit_rows` logic to always validate rows

Previously, after 'commit()`, rows in need of merging had status `WAITING`.
That seemed like a bad policy, and also caused a row with both an errror and a merge
requirement to be displayed in the "Merge Required" tab rather than the "Errors" tab
after being committed.