[fractal] Give panic message when gtksing is missing

[Julian Sparber <jsparber src gnome org>]

commit c8843e98d5a041d7a89065dadd3a33586859406d
Author: Sonja Heinze <sonjaleaheinze gmail com>
Date:   Wed Apr 1 22:23:48 2020 +0200

    Give panic message when gtksing is missing

 fractal-gtk/src/widgets/inline_player.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/fractal-gtk/src/widgets/inline_player.rs b/fractal-gtk/src/widgets/inline_player.rs
index fadb42b0..496de462 100644
--- a/fractal-gtk/src/widgets/inline_player.rs
+++ b/fractal-gtk/src/widgets/inline_player.rs
@@ -247,7 +247,8 @@ pub struct VideoPlayerWidget {
 impl Default for VideoPlayerWidget {
     fn default() -> Self {
         let dispatcher = gst_player::PlayerGMainContextSignalDispatcher::new(None);
-        let sink = gst::ElementFactory::make("gtksink", None).unwrap();
+        let sink = gst::ElementFactory::make("gtksink", None)
+            .expect("Missing dependency: element gtksink is needed (usually, in gstreamer-plugins-good or in 
gst-plugin-gtk).");
         let renderer = gst_player::PlayerVideoOverlayVideoRenderer::new_with_sink(&sink).upcast();
         let player = gst_player::Player::new(
             Some(&renderer),