library: some providers has bug in provider.song_get :( (#773)

feeluown/library/library.py

@@ -389,6 +389,8 @@ def _model_upgrade(self, model):
             elif e.reason is ModelNotFound.Reason.not_supported:
                 model.state = ModelState.cant_upgrade
             raise
+        if upgraded_model is None:  # some provider does not implement
+            raise ModelNotFound(f'{provider} implementation error, it returns None :(')
         return upgraded_model
 
     # --------

feeluown/library/provider.py

@@ -41,6 +41,9 @@ def identifier(self):
     def name(self):
         """provider name"""
 
+    def __str__(self):
+        return f'provider:{self.identifier}'
+
     @contextmanager
     def auth_as(self, user):
         """auth as a user temporarily