Fix accessing a discussion without being a member

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/graphql/resolvers/discussion.ex

@@ -60,6 +60,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Discussion do
       {:ok, discussion}
     else
       nil -> {:error, dgettext("errors", "Discussion not found")}
+      {:member, false} -> {:error, :unauthorized}
     end
   end