Maintenance: Update side-nav.yml

[iveloff-developer]

src/content/language/loops.md and src/content/language/branches.md both refer to src/content/language/error-handling.md as a part of Control Flow in Dart that seems logical. To make docs little more consistent, it's a good idea to move 'Error Handling' topic under 'Control Flow' header inside side navigation panel.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• This PR doesn't contain automatically generated corrections or text (Grammarly, LLMs, and similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn't use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Code changes should generally follow the Dart style guide and use dart format.
• Updates to code excerpts indicated by <?code-excerpt need to be updated in their source .dart file as well.

[MaryaBelanger]

/gcbrun

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 6fc7095):

https://dart-dev--pr6202-main-8zoaysz0.web.app

[parlough]

This makes sense to me, but I'll let @MaryaBelanger decide.

Thanks so much for thinking about this and the pull request!

[parlough]

/gcbrun

[MaryaBelanger]

Good idea! Thanks @iveloff-developer