[xmonad] [patch] output exitcode when no xmonad.error output

Hi, If xmonad is running without ghc installed for some reason and one tries to restart it (mod-q), the recompile error dialog doesn't contain any useful error message from execv (it fails silently when it can't find the command). Here is a small patch to output the exitcode when compilation fails and xmonad.error is empty. Jens