fix: Don't panic if token is still valid

1. `None` from `update_token` should only mean "no update needed", should not be used for errors. We now panic when worker channel is not set
2. Callers should correctly handle `None` result

src/spotify_api.rs

@@ -117,8 +117,7 @@ impl WebApi {
                 }
             }))
         } else {
-            error!("worker channel is not set");
-            None
+            panic!("worker channel is not set");
         }
     }