[Xmonad] darcs patch: catch exceptions when calling user-written code.

This is an amended version of this patch (with its long comment modified to match), since sjanssen didn't like the whenJust catching. David Thu Oct 11 21:33:05 EDT 2007 David Roundy <droundy@darcs.net> * catch exceptions when calling user-written code. This is a minimal approach that only catches error in actual user-written code.