log errors on executeFile in restart

I found it difficult to track down a problem in the restart code where xmonad was silently not restarting. This will log the error to stderr, which should show up in .xsession-errors

XMonad.hs

@@ -150,7 +150,7 @@ restart mprog resume = do
     prog <- maybe (io $ getProgName) return mprog
     args <- if resume then gets (("--resume":) . return . show . windowset) else return []
     io $ catch (executeFile prog True args Nothing)
-               (const $ return ()) -- ignore executable not found exception
+               ( (hPutStrLn stderr). show  ) -- print executable not found exception
 
 -- | Run a side effecting action with the current workspace. Like 'when' but
 whenJust :: Maybe a -> (a -> X ()) -> X ()