- When you _throw_ an error at a cell's top level, the runtime will catch it and mark the cell as having errored - When you _return_ an error object from a cell, the error becomes the cell's "valid" value. What you see then is merely the formatting applied by the Inspector (including the browser specific stack trace). - When you _return_ a promise that errors, the runtime will resolve the promise and catch the error, again marking the cell as having errored - When you _await return_ a promise that errors, the exact same thing happens. The only difference is the position in the stack trace where the error is encountered. For logging/debugging purposes it can make sense to await a promise and only return its result (even though an async function's return value is always a promise).

You don't pass an error here though, because you don't return the promise. Like with async errors in event handlers the Runtime has no way to detect this one.

A few more examples ... your first example can be simplified to ~~~ new Promise((resolve, reject) => { reject(new Error("Bad!")) }) ~~~ Notice how the error marker is at the right position, because the Runtime is able to parse the stack trace. Compare it to: ~~~ new Promise((resolve, reject) => { reject("Bad!")}) ~~~ Likewise, your second example can be simplified to: ~~~ new Promise((resolve, reject) => { reject(new Error("Bad!"))}), true ~~~ Now compare what happens if you await: ~~~ (await new Promise((resolve, reject) => { reject(new Error("Bad!"))}), true) ~~~