Merge pull request matrix-org#1799 from matrix-org/travis/error1

Actually print IndexedDB command error if there is one

src/store/indexeddb-remote-backend.ts

@@ -185,7 +185,7 @@ export class RemoteIndexedDBStoreBackend implements IIndexedDBBackend {
                 def.reject(error);
             }
         } else {
-            logger.warn("Unrecognised message from worker: " + msg);
+            logger.warn("Unrecognised message from worker: ", msg);
         }
     };
 }