Allow serving custom error page (#393)

It might be useful for those self-hosting a Codeberg Pages instance to be able to serve a different error page than the one embedded with go:embed

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/393
Co-authored-by: Gnarwhal <git.aspect893@passmail.net>
Co-committed-by: Gnarwhal <git.aspect893@passmail.net>

flake.lock

@@ -5,11 +5,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1726560853,
-        "narHash": "sha256-X6rJYSESBVr3hBoH0WbKE5KvhPU5bloyZ2L4K60/fPQ=",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "c1dfcf08411b08f6b8615f7d8971a2bfa81d5e8a",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
@@ -20,8 +20,8 @@
     "nixpkgs": {
       "locked": {
         "lastModified": 0,
-        "narHash": "sha256-8fnFlXBgM/uSvBlLWjZ0Z0sOdRBesyNdH0+esxqizGc=",
-        "path": "/nix/store/rqicl40jqw5d93l43s1xq0bm6yy0gpx4-source",
+        "narHash": "sha256-x07g4NcqGP6mQn6AISXJaks9sQYDjZmTMBlKIvajvyc=",
+        "path": "/nix/store/2w8kz6zh3aq80f1dypiin222fry1rv51-source",
         "type": "path"
       },
       "original": {