Determine error view and message based on type

...not based on status code.

To simplify this logic, we now use the same error "type" both when
routes are not found and specific models are not found. One exception is
ours, one is from Laravel, but for the purposes of error handling they
should be treated the same.

Fixes flarum/core#1641.

framework/core/src/Http/Exception/RouteNotFoundException.php

@@ -18,6 +18,6 @@ class RouteNotFoundException extends Exception implements KnownError
 {
     public function getType(): string
     {
-        return 'route_not_found';
+        return 'not_found';
     }
 }