core: Warn if track isn't playable

docs/changes.rst

@@ -10,6 +10,11 @@ v0.13.0 (in development)
 
 (in development)
 
+**Core**
+
+- Log a warning if a track isn't playable to make it more obvious that backend
+  X needs backend Y to be present for playback to work.
+
 **Spotify backend**
 
 - Let GStreamer handle time position tracking and seeks. (Fixes: :issue:`191`)

mopidy/core/playback.py

@@ -397,13 +397,14 @@ class PlaybackController(object):
             self.state = PlaybackState.PLAYING
             backend = self._get_backend()
             if not backend or not backend.playback.play(tl_track.track).get():
-                # Track is not playable
+                logger.warning('Track is not playable: %s', tl_track.track.uri)
                 if self.random and self._shuffled:
                     self._shuffled.remove(tl_track)
                 if on_error_step == 1:
                     self.next()
                 elif on_error_step == -1:
                     self.previous()
+                return
 
         if self.random and self.current_tl_track in self._shuffled:
             self._shuffled.remove(self.current_tl_track)