Improve default error message for optimize/repair table

sql/sql_table.cc

@@ -2388,12 +2388,17 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
     }
 
     default:				// Probably HA_ADMIN_INTERNAL_ERROR
+      {
+        char buf[ERRMSGSIZE+20];
+        uint length=my_snprintf(buf, ERRMSGSIZE,
+                                "Unknown - internal error %d during operation",
+                                result_code);
         protocol->store("error", 5, system_charset_info);
-      protocol->store("Unknown - internal error during operation", 41
-		      , system_charset_info);
+        protocol->store(buf, length, system_charset_info);
         fatal_error=1;
         break;
       }
+    }
     if (fatal_error)
       table->table->s->version=0;               // Force close of table
     else if (open_for_modify)