Fix: If no sunset/sunrise calculation is possible a wrong value is shown

This is catched now and a proper message is shown in the web UI. Also the mode is set to day.

include/SunPosition.h

@@ -12,6 +12,7 @@ public:
     void loop();
 
     bool isDayPeriod();
+    bool isSunsetAvailable();
     bool sunsetTime(struct tm* info);
     bool sunriseTime(struct tm* info);
 
@@ -20,6 +21,7 @@ private:
 
     SunSet _sun;
     bool _isDayPeriod = true;
+    bool _isSunsetAvailable = true;
     uint _sunriseMinutes = 0;
     uint _sunsetMinutes = 0;