Notify admin user on cron failure and dispaly a warning/error on the cron jobs table

It would be a good idea to notify the admin user on cron failure (i.e. when the execution of vtigercron.php causes an exception or fatal error) and also display it on the crons table. Currently, if there is an error, the cron job is marked as "running" when in fact it is not running because an error ocurred.

See here: http://vtiger-crm.2324883.n4.nabble.com/Vtigercrm-developers-Time-Interval-workflow-exhausts-memory-td24039.html

Also, when a task is running (from the previous cronjobs execution), it should not run again.