Handle error

[Rich-Harris]

Closes #1857

#2122 with a few tweaks — updated docs, and reinstated handle_error. Despite the naming similarity, they're separate things — handleError is the user's code, handle_error is what the framework does (which includes calling the user's code). It's better to avoid conflating them, even if the framework isn't doing much beyond what the user does (i.e. just manipulating error.stack)

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: ad10858

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[benmccann]

This PR looks to be accidentally reverting #2178

[Rich-Harris]

yeah, there was a conflict. i was going to tweak it anyway, 'This function runs on every rendering' reads to me like a typo — sorry!

[benmccann]

looks good to me other than that

just FYI, there was also a highlight that was removed besides the rendering / request wording

handle runs when a page is rendered. If a page is prerendered, handle will not run again when the user requests the page. If you would like to run some functionality on each request, see the documentation for your adapter.