Rely on the default error page triggering for 404s

It is not immediately obvious how to avoid the error handler making a
different 404 response when the application code returns an explicit 404
page. Since we were already replying "Resource not found" in all cases
except one where we reply "File not found" not much is lost by relying
on the error handler behavior.

lib/builder_web.ml

@@ -64,9 +64,6 @@ let or_error_response r =
   let* r = r in
   match r with
   | Ok response -> Lwt.return response
-  | Error (text, `Not_Found) ->
-    Views.resource_not_found ~text
-    |> string_of_html |> Dream.html ~status:`Not_Found
   | Error (text, status) -> Dream.respond ~status text
 
 let default_log_warn ~status e =

lib/views.ml

@@ -206,16 +206,6 @@ let artifact
     txtf " (%a)" Fmt.byte_size size;
   ]
 
-let resource_not_found ~text =
-  [
-    H.h2 ~a:[ H.a_style "padding-top: 33vh" ]
-      [ txtf "This resource does not exist" ];
-    H.p [
-      H.txt @@ Fmt.str "Error: '%s'" text
-    ];
-  ]
-  |> layout ~title:"Resource not found"
-
 let page_not_found ~target ~referer =
   [
     H.h2 ~a:[ H.a_style "padding-top: 33vh" ]