Plane: Display Fence Breach message in GCS

Send a "Geofence breach" message to the Ground Control Station. Without this when the fence is breached and if anything happens as a result, such as RTL, it will happen silently.

ArduPlane/fence.cpp

@@ -51,6 +51,10 @@ void Plane::fence_check()
         return;
     }
     
+     if(new_breaches && plane.is_flying()) {
+         GCS_SEND_TEXT(MAV_SEVERITY_NOTICE, "Fence Breached");
+     }
+
     if (new_breaches || orig_breaches) {
         // if the user wants some kind of response and motors are armed
         const uint8_t fence_act = fence.get_action();