Add example for error handling in ExecutionContext

[som-snytt]

It was helpful to me to work out this example, as a refresher. Therefore, it may be helpful to beginners, in case there are still beginners out there using Future for the first time.

The existing words are correct, but might require a few hours of experimentation (or debugging) to comprehend.

[som-snytt]

The failure is unrelated

For the Links > Internal check, the following failures were found:

* At ./_site/sips/quote-pattern-type-variable-syntax.html:604:

  internally linking to Explicit-type-variables, which does not exist

Tried rebasing.

[som-snytt]

The example should be specialized for Scala 3. (In particular, nonlocal return.)

[julienrf]

Hey, indeed the failure was unrelated to your PR, I fixed it on main.

[julienrf]

Thank you very much @som-snytt for adding this clarification! I have left a few comments.

[julienrf]

Hey @som-snytt, I took the liberty to make some changes to your initial PR, I hope this is fine with you. What do you think of the result?

Here is the rendering of the Scala 3 version:

[som-snytt]

Thanks. I see indentation doesn't matter after all. (I wasn't sure.) I don't have approval privilege on this repo, but I approve this message.

[julienrf]

Thank you for the review! I’ve updated the imports according to your feedback. What do you think of the result?