twister: Alleviate DeviceHandler serial port TOCTOU

The DeviceHandler class method monitor_serial() has time-of-check
to time-of-use (TOCTOU) race condition possible on its serial port
which might be closed and its file descriptor cleared in between
of isOpen() and is_waiting status checks.
The issue is alleviated with controlled exception handling.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>