ShellPrompt: check for executables and better error handling
ShellPrompt: check for executables and better error handling

Code contributed by Spencer (basically I just removed FilePath
depenency).

darcs-hash:20071007110133-32816-49cab7bd033569a16c55a3d1bfde3aba8c9fc3ee

kmels/xmonad-launcher,kmels/xmonad-launcher