Maybe there’s a better way to do this, but I feel like
throw new Error(...) can be lossy in terms of the representation - it can be pretty hard to read long wrapping red text on errors which contain information about some sizable object. What if the standard library provided a JSONError or the like that, if thrown, let you interact with the response, as you can for objects in an unerring cell, and gave some other indication that the result is an error (the red/stacktrace/jump button are probably clear enough)?
Also, along the same lines - it would be cool if the sidebar showed red dots not just for the erring cell, but for the ones along the stacktrace (or maybe shades of orange/yellow on a gradient indicating how far back in the stack each one is).
Anyone else have this problem?