Spotifysync crashes when tracks do not have audio-features
Some tracks in Spotify do not have audio-features. This causes `spotifysync` to crash:

```
spotify: Processing 48/471 tracks - Arijit Singh, Arko - OM - Rashtra Kavach - Seher
error: Spotify API error:
{
  "error" : {
    "status" : 404,
    "message" : "analysis not found"
  }
}
URL:
https://api.spotify.com/v1/audio-features/5pjSpt2mstf5JTf46FbT48
params:
None
```

`spotifysync` should gracefully handle such errors. Maybe we need a try-except block [here](https://github.com/beetbox/beets/blob/master/beetsplug/spotify.py#L622). Thoughts?
diff --git a/beetsplug/spotify.py b/beetsplug/spotify.py
index 42e20c828..a9c8368b0 100644
--- a/beetsplug/spotify.py
+++ b/beetsplug/spotify.py
@@ -35,6 +35,10 @@ from beets.plugins import BeetsPlugin, MetadataSourcePlugin
 DEFAULT_WAITING_TIME = 5
 
 
+class SpotifyAPIError(Exception):
+    pass
+
+
 class SpotifyPlugin(MetadataSourcePlugin, BeetsPlugin):
     data_source = 'Spotify'
 
@@ -189,6 +193,9 @@ class SpotifyPlugin(MetadataSourcePlugin, BeetsPlugin):
                     seconds.', seconds)
                 time.sleep(int(seconds) + 1)
                 return self._handle_response(request_type, url, params=params)
+            elif response.status_code == 404:
+                raise SpotifyAPIError("API Error {0.status_code} for {1}"
+                                      .format(response, url))
             else:
                 raise ui.UserError(
                     '{} API error:\\n{}\\nURL:\\n{}\\nparams:\\n{}'.format(
@@ -621,6 +628,9 @@ class SpotifyPlugin(MetadataSourcePlugin, BeetsPlugin):
             item['spotify_track_popularity'] = popularity
             audio_features = \\
                 self.track_audio_features(spotify_track_id)
+            if audio_features is None:
+                self._log.info('No audio features found for: {}', item)
+                continue
             for feature in audio_features.keys():
                 if feature in self.spotify_audio_features.keys():
                     item[self.spotify_audio_features[feature]] = \\
@@ -639,7 +649,9 @@ class SpotifyPlugin(MetadataSourcePlugin, BeetsPlugin):
 
     def track_audio_features(self, track_id=None):
         """Fetch track audio features by its Spotify ID."""
-        track_data = self._handle_response(
-            requests.get, self.audio_features_url + track_id
-        )
-        return track_data
+        try:
+            return self._handle_response(
+                requests.get, self.audio_features_url + track_id)
+        except SpotifyAPIError as e:
+            self._log.debug('Spotify API error: {}', e)
+            return None
['beetsplug/spotify.py']