Ensure handleError hook is called for shadow endpoint errors

[georgecrawford]

Fixes #3807

I'm honestly not sure this is the right place, or the right treatment for errors thrown in shadow endpoints, but it seems wrong that they weren't being passed to the handleError hook. I added a failing test, and tracked down a possible fix location. The test still fails as the assertion for page.textContent('#message') is getting the error message, but expecting an empty string. I don't know what's desired here (I just copied the 'error in endpoint' test above), so the failure might indicate the fix is more complex than I thought. Anyway, it's a start!

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Latest commit: 8411533

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[Rich-Harris]

Thanks. handleError is only called for unexpected errors — returning { status: 500 } shouldn't result in handleError running. This distinction wasn't covered by the existing tests, so I added some and added the handle_error call where it needs to go: 8411533

[georgecrawford]

Thanks @Rich-Harris. I was pretty sure I was close but not quite right on this one!