fix: errors thrown from iterables now properly propagated

[benlesh]

Resolves an issue where if you were trying to use an iterator to do something like examine and flatten an array in one go, it would not properly handle the error, particularly in async cases:

const requestData$: Observable<void> = buttonClicksOrSomething$;

requestData$.pipe(
   concatMap(() => {
      const dataStream$: Observable<Data[]> = getDataStream();

       return dataStream$.pipe(concatMap(function* (rows) {
           for (const row of rows) {
              if (isInvalidRow(row)) {
                  throw new Error('invalid row');
              }
              yield row;
           }
       })
   }
)

Closes #5436

[cartant]

LGTM