Fix crash on failed lookup of URI from Spotify

docs/changes.rst

@@ -53,6 +53,8 @@ to this problem.
     normal playlists. More permanent fix requires support for checking playlist
     types in pyspotify (see :issue:`62`).
 
+  - Fix crash on failed lookup of track by URI. (Fixes: :issue:`60`)
+
 - Local backend:
 
   - Add :command:`mopidy-scan` command to generate ``tag_cache`` files without

mopidy/backends/spotify/library.py

@@ -22,7 +22,7 @@ class SpotifyLibraryProvider(BaseLibraryProvider):
             # playlists.
             return SpotifyTranslator.to_mopidy_track(spotify_track)
         except SpotifyError as e:
-            logger.warning(u'Failed to lookup: %s', uri, e)
+            logger.debug(u'Failed to lookup "%s": %s', uri, e)
             return None
 
     def refresh(self, uri=None):