Comment 1 for bug 1223798

Hmm, actually there are a couple of cases here

  - The background chosen by the user from the application is invalid
    + Maybe refuse the selection and keep what we had previously.
    + Or fall back to Unity's default.
  - Externally (via gnome-control-center or gsettings or whatever) an invalid
    URL is set. Could be one of these 'timed' xml files or something else
    altogether.
    + Trust the shell to do the right thing, perhaps? In a GNOME-like
      environment the xml file will be handled correctly and we don't want to
      unilaterally overrule this in u-s-s even if we don't know how to show it.
      But then what do we display? Not sure.

Probably a Q. for design in there.