[pgAdmin4][RM#3238] Proper error handling when connection will belost to server.

PFA patch for proper error handling when connection will be lost to server. I have created a common function alertify.pgRespErrorNotify in web/pgadmin/static/js/alertify.pgadmin.defaults.js on the lines of existing function alertify.pgNotifier. The function is called in error callback of $.ajax call. The XHR is passed and handled in the function itself. Please note that I have used JSON.parse and not $.parseJSON as it will be deprecated in jQuery 3.x and thus removing many $.parseJSON calls.

Request you to kindly review.