Inconsistent handling of "not found" errors

[joshi-monster]

Hi!

My original goal was to get a custom 404 page to work. I have found a partial solution for now (edit the netlify build command to copy a static public/404.html to dist after elm-pages build), but I have found a bunch of unexpected behaviors related to 404 handling, which I wanted to document here.

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Depending on "how" a 404 error is produced, different things will happen in elm-pages v3.

• If a request is made to the dev server, a 404 error with this message shown (from Pages.Internal.NotFoundReason): "No route found for /asdf Did you mean to go to one of these routes:"
• In production, this always changes to "No route found for /____elm-pages-internal____/404 Did you mean to go to one of these routes:" in the generated 404.html file.
• Using Netlify redirect rules for example, you can load an elm-pages app with a url unknown to elm-pages. In that case, you will get the following error, originating from the generated Main.elm: "This page could not be found."
• Next, I tried to use a top-level SPLAT_.elm. This fails with a compiler error in the generated Route.elm: "The nth pattern is reduntant: Any value with this shape will be handled by a previous pattern, so it should be removed." (see Issue with only catch-all routing #397)
• a public/404.html file will be overridden by elm-pages build.

Additionally, we now have an ErrorPage.elm module in v3, which would be another way of rendering a 404! Unfortunately, using a server-rendered page that just 404s for everything also does not work because of the SPLAT error.

What I think should happen

Since we now have this ErrorPage.elm module, I think it should be used not only for server-rendered pages, but for all errors in general. Errors in pre-rendered pages are caught at build-time. 404s can always happen though!

The ErrorPage module is already required to expose a notFound constructor. In all cases where there is currently a error built-in from elm-pages shown, it should instead use this constructor and switch to the error page.

In the generated 404.html page, this module should also be used.