gateway: print more clear message when slabstore has higher index than gateway's slabstore

src/service/gateway.rs

@@ -228,7 +228,7 @@ impl GatewayClient {
         if last_index < local_last_index {
             return Err(Error::SlabsStore(
                 "Local slabstore has higher index than gateway's slabstore.
-                 Delete slabstore database \"darkfid_client.db\"."
+                 Run \" darkfid -r \" to refresh the database."
                     .into(),
             ));
         }