Spotlight widget shouldn't assume that image exists when rendering

Currently, the Spotlight widget assumes that each image exists when rendering. This is a safe assumption under normal circumstances because it's a required field on the settings form.

However, it's possible to remove files by going to "Content" -> "Files" (/admin/content/file) and removing them after the fact!

Doing so will cause a load of PHP notices and warnings:

Notice: Undefined index: description in theme_panopoly_spotlight_view() (line 375 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).
Notice: Undefined index: link in theme_panopoly_spotlight_view() (line 376 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).
Notice: Undefined index: alt in theme_panopoly_spotlight_view() (line 377 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).
Notice: Undefined index: uuid in theme_panopoly_spotlight_view() (line 381 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).
Notice: Trying to get property of non-object in theme_panopoly_spotlight_view() (line 382 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).
Warning: array_flip(): Can only flip STRING and INTEGER values! in DrupalDefaultEntityController->load() (line 173 of /var/aegir/platforms/panopoly-dev/includes/entity.inc).
Notice: Trying to get property of non-object in theme_panopoly_spotlight_view() (line 388 of /var/aegir/prj/panopoly/panopoly_widgets/panopoly_widgets.spotlight.inc).

It would be better if we skipped rendering slides that had missing images. Users can then edit the Spotlight widget and replace the image or delete the slide if necessary.