ErrorPage shows "NotFound" regardless of error type

[adamdicarlo]

The ErrorPage template always shows a NotFound, even when there's an internal error, such as in a BackendTask on a server rendered page.

See

• https://github.com/dillonkearns/elm-pages-starter/blob/d87c6c7d94672e40bb44bf7f799342a72d0b75f4/app/ErrorPage.elm#L57
• https://github.com/dillonkearns/elm-pages/blob/57dccef4334e264659af53465de786028e9566d9/generator/template/app/ErrorPage.elm#L57-L70