Fasty is coming with a default error page ... A nice one but hey, I guess you want to display the content you want instead of a generic page.
Go to your settings and edit the General Settings like that :
{
...
"error_404": "my-404error-page",
"error_500": "my-500error-page"
...
}
Just define the slug of the page you want to display and it will use it. If you don't define any page it will fallback to the default error pages.
As you can see, error 500 pages can be defined as well :)
I guess you already know that Fasty is a fast CMS, the code base is very low (960 LOC for the frontend). Everything is blazing fast ! You can build any kind of application within Fasty !
Even using it in development environment mode is fast !