The previous check relied upon the (undocumented) return value of
XChangeProperty to determine the exit status; at least, on my gentoo system,
the function seems to return 1 even when it fails to change the property
due to contrived, and invalid, parameters. However, setting an error
handler allows proper detection of this case. So, now the return status
properly indicates whether or not the property was changed (but not
whether fluxbox liked the command!).
