Cover error handling: exceptions, try/catch, and when to throw

**Which module(s) and week(s) does this change affect?**
Module(s): ITP, earlier than Data Flows (e.g. Structuring Data or Data Groups), plus Data Flows Sprint 3 for the async follow-up
Week(s): To be decided

**What is the work that needs to be done?**

We want to properly cover error handling:

- What exceptions are and what happens when one goes unhandled
- What try/catch is and how to use it
- When you should throw vs handle the error another way (return a value, log, recover, re-throw)
- How to handle errors well: catching at the right level, not swallowing errors, giving users/callers useful information

There is an existing block, `common-content/en/module/js3/catch`, which covers try/catch around async fetch calls. It was deliberately removed from Data Flows Sprint 3 prep in 4f8508cd (#1111), with this rationale in the commit message:

> 1. We haven't actually taught `try-catch` anywhere, so we can't just rely on transferring "it's the same as you already know, just async".
> 2. There are already _way_ too many novel concepts in Data Flows Sprint 3, and it's already very hard to follow from scratch.
>
> If we add `try-catch` earlier in the course (which I think we should), we should re-add this.

So the work is likely two parts:

1. Introduce error handling fundamentals earlier in the course, in a synchronous context
2. Re-add the `js3/catch` block (or an updated version) to Data Flows Sprint 3, building on that foundation

**Why is this work important to do?**

Trainees currently learn fetch, promises, and async/await without ever being taught what to do when things fail. The removal in #1111 was explicitly conditional on teaching this earlier, which hasn't happened yet.

**Who might need to know about this change?**

@illicitonion