The "Files" slideshow should better handle broken previews

[oparoz]

The files app is not telling plugins whether a preview is working or not, so we have to try to load it to see what happens.

If we encounter a broken preview, we should let the user know and remove it from the list of images for the currently loaded slideshow

A good test case is to enable the SVG provider. Most moderately complex SVGs won't have previews because the provider is crippled for security reasons.
The thumbnail should be blank and the slideshow will fail to get an image.