Document error handling best practices

Describe the problem

There are currently a variety of tools available for error handling in SvelteKit:

• +error.svelte pages
• The error helper
• <svelte:boundary> and its failed snippet
• Vanilla Javascript try / catch logic
• handleValidationError hooks

These all have their place, but the documentation lacks an overview of when to use which tool.

I was surprised, for example, that <button onclick={() => {throw Error("say what?")}>Yo</button> wasn't caught by a <svelte:boundary>.

Describe the proposed solution

A best practices page for error handling, providing an guided tour of each of these tools. The page would not go into depth on each tool, but rather focus on explain when and how to use each tool your project.

Start by reading the existing documentation for +error.svelte pages, the error helper, svelte:boundary, vanilla JavaScript try/catch, and handleValidationError. Add an error-handling best-practices page that explains when and how each tool is used, including the documented boundary behavior for errors thrown in button onclick handlers.