removed error messages when xclip not installed
removed error messages when xclip not installed

When you run gist on a Linux server without xclip 
installed error messages are displayed on the 
screen -- this patch discards the error messages
printing of xclip is not installed

Signed-off-by: Chris Wanstrath <711c73f64afdce07b7e38039a96d2224209e9a6c@ozmm.org>
dLobatog/gist,ConradIrwin/gist,eLobato/gist,retrography/gist,Jwata/gist-old,roxyboy/jist,ConradIrwin/jist,zmwangx/gist,roxyboy/gist,ConradIrwin/gist,mofodox/gist,bchase/gist,mattn/gist,flores/gist,ryanbradynd05/gist,defunkt/gist