[fix] deeply-nested error components render with correct layout

[rmunn]

Under some circumstances, a deeply-nested error component could render with a layout from a parent folder instead of the layout from the error component's own folder. This commit fixes that bug, so that error components are correctly matched up with layout components in the same folder even when deeply nested.

Fixes #2383.

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[rmunn]

The root cause was modifying error_stack in place in the create_manifest_data function. Under certain conditions, which #2383 reproduces, error_stack could contain [ root_error, undefined, undefined, nested_error ] and the correct logic would be to remove precisely one undefined from that array, but not two. But because error_stack was being modified in place rather than a copy being made and the copy modified, the first pass through the loop would correctly remove a single undefined to produce [ root_error, undefined, nested_error ], but the second pass through the loop would again remove a single undefined to produce [ root_error, nested_error ]. The second pass through the loop would therefore produce an error stack that was out of sync with the component stack, and the error would then be rendered in the wrong layout.

By making a copy of the error_stack array and modifying the copy, both passes through the loop start with [ root_error, undefined, undefined, nested_error ] and therefore both passes correctly produce [ root_error, undefined, nested_error ], which lines up the nested_error component correctly with the layout it's supposed to be rendered with.

[benmccann]

thanks!