Some cells (e.g. $macc_v2) are marked evaluable, but will raise an abort if called with `CellTypes::eval()`.
Instead of falling through to the abort, we can pass a pointer to a boolean to check for errors.
Use said check to catch `CellTypes::eval()` errors and treat them as unevaluable but otherwise continue.
Reflows the series of if checks into `if ... else if ... else` so that we can check for errors and set state in one place.
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