Shadow Endpoint errors don't hit handleError in SSR

[georgecrawford]

Describe the bug

When an error is throw in an endpoint get() method, it's not passed to handleError if called as a shadow endpoint via SSR.

Calling the endpoint manually with fetch(), or fetching the shadow endpoint by means of a client-side navigation all trigger handleError as expected.

I believe this is the problem I'm trying to wrap my head around in this discussion.

Reproduction

I've made a simple reproduction here: https://github.com/georgecrawford/sveltekit-shadow-endpoint-handle-error (also available live in StackBlitz here: https://stackblitz.com/edit/node-wy166v?file=src%2Fhooks.js&terminal=dev).

There are two endpoints, both of which throw an Error inside get(). When the endpoint is called from a load() function, both SSR and browser routing will cause the error to be handled by handleError. However, when an endpoint is called as a shadow endpoint, the SSR version doesn't hit handleError (whereas the browser-routed shadow load does as expected).

Logs

No response

System Info

System:
    OS: macOS 11.6.2
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 202.62 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 16.4.0 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 7.19.0 - /usr/local/bin/npm
    Watchman: 2021.09.06.00 - /usr/local/bin/watchman
  Browsers:
    Chrome: 98.0.4758.80
    Chrome Canary: 100.0.4878.0
    Firefox: 92.0.1
    Safari: 15.2
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.17
    @sveltejs/kit: next => 1.0.0-next.262
    svelte: ^3.44.0 => 3.46.4

Severity

blocking an upgrade

Additional Information

No response