Notify user when an error occurs

https://github.com/SimpleMobileTools/Simple-Calendar/issues/1601

app/src/main/kotlin/com/simplemobiletools/calendar/pro/helpers/CalDAVHelper.kt

@@ -152,7 +152,8 @@ class CalDAVHelper(val context: Context) {
         val existingEvents = try {
             context.eventsDB.getEventsFromCalDAVCalendar("$CALDAV-$calendarId")
         } catch (e: Exception) {
-            ArrayList()
+            context.showErrorToast(context.getString(R.string.unknown_error_occurred))
+            return
         }
 
         existingEvents.forEach {