Observable errors

So what happens when something goes wrong within an Observable stream? Obviously, we will need a mechanism to catch these errors, so that we can do something sensible with them. As an example of a faulty Observable stream, consider the following code:

interface IValue {
    value: number
}
interface INestedObj {
    id?: IValue;
}
const objEmit : Observable<INestedObj> = of(
    { id: { value: 1 } },
    {},
    { id: { value: 2 } }
);

Here, we start with two interfaces, named IValue and INestedObj. The IValue interface has a property named value of type number, and the INestedObj has a single optional parameter named id of type IValue. We then create an Observable named objEmit that emits three values. The first value has the nested structure described by the INestedObj interface, and the second is a blank object. Now consider the following Observable stream:

const returnIdValue = objEmit.pipe(
    map((value: INestedObj) => {
        return...