Log low-level reason and idle time on master failure

If the connection to the master is lost, knowing what type of an error
caused the call to handleError helps deduce what was the real reason for
it. Logging the idle time of the connection helps detect when the
wait_timeout of a connection is exceeded.

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -1465,7 +1465,8 @@ static int backend_write_delayqueue(DCB* dcb, GWBUF* buffer)
 
     if (rc == 0)
     {
-        do_handle_error(dcb, ERRACT_NEW_CONNECTION, "Lost connection to backend server.");
+        do_handle_error(dcb, ERRACT_NEW_CONNECTION,
+                        "Lost connection to backend server while writing delay queue.");
     }
 
     return rc;