MWL#121-125 DS-MRR improvements

- Address Monty's review feedback, part 1
- Fix buildbot failure

sql/sql_join_cache.cc

@@ -2123,7 +2123,7 @@ enum_nested_loop_state JOIN_CACHE::join_matching_records(bool skip_last)
 
   /* Prepare to retrieve all records of the joined table */
   if ((error= join_tab_scan->open())) 
-    goto finish;
+    goto finish; /* psergey-note: if this returns error, we will assert in net_send_statement() */
 
   while (!(error= join_tab_scan->next()))   
   {