Return a 404 if a distro doesn't exist

uitwww/download.py

@@ -183,6 +183,9 @@ class Downloads:
 
         @bp.route("/<distro>")
         def landing(distro):
+            if distro not in self.config["distros"]:
+                flask.abort(404)
+
             return flask.render_template(
                 "download/landing.html",
                 distro_name = distro,