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

[Jens Petersen]

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