fix exception when file does not exist

fixes https://git.clan.lol/clan/clan-core/issues/488

pkgs/clan-cli/tests/test_static_files.py

@@ -0,0 +1,10 @@
+import pytest
+from api import TestClient
+
+
+@pytest.mark.impure
+def test_static_files(api: TestClient) -> None:
+    response = api.get("/")
+    assert response.headers["content-type"] == "text/html; charset=utf-8"
+    response = api.get("/does-no-exists.txt")
+    assert response.status_code == 404