Add some rudimentary error handling to users kcm

Don't fail silently when an error occurs otherwise the kcm just looks broken. Show at least an inline message so the user knows that something went wrong. The comment was wrong. We can distinguish why an error occured. Furthermore, even if it is a permission denied error we do not know if the user canceled the action or if it failed for some other reason so also treat that as an error. BUG:425036 CCBUG:426932 FIXED-IN:5.20 CCMAIL:uhhadd@gmail.com