show warning on missing plugin
Currently totem reacts to opening YouTube videos without the needed plugins just by showing an error message on the console, whereas an inexpressive black screen (or a confusing thumbnail of the last viewn vid) is shown to the user.
r@aragorn ~> env G_DEBUG=all totem
** Message: 15:40:21.025: Missing plugin: gstreamer|1.0|totem|text/html-Decoder|decoder-text/html (text/html-Decoder)
** Message: 15:40:21.027: Automatic missing codec installation not supported (helper script missing)