newsticker: Prevent multiple "Could not download..." messages. Fixes bug#19166.

newsticker: Prevent multiple "Could not download..." messages.  Fixes bug#19166.

* lisp/net/newst-backend.el (newsticker--get-news-by-url-callback):
Pass correct status to `newsticker--sentinel-work'.
(newsticker--sentinel-work): Use "newsticker--download-error" as
guid in order to prevent multiple "Could not download..."
messages.  Fixes bug#19166.

* lisp/net/newst-treeview.el (newsticker--treeview-item-show): Check
window liveliness before measuring its width.