Add an error handling code for network connection and server problems in TicketDownload class within TicketFragment.

1. showErrorToast function in TicketDownload class
    shows toast message to the screen

2. Call cancel(true) function when catching error while connecting to server in doInBackground method. cancel function cancel any processing and move to onError handler where calls showErrorToast.

3. Check if network is connected in onPreExecution method. If failed, call showErrorToast function with message, "network connection failed".

4. Change isConnected method to static function in NetworkConnector class to get access to any function.