Which version of Qlik Sense are you on? Regarding your questions:
a) I could not find file which captured the failure. -- The error files you can find on Scheduler node at "E:\Qlik\Sense\Log\Script" or in archived logs (E:\Qlik\Sense\Repository\Archived Logs) at central node.
b) Looking at the content looks like task failed after 30 minutes. You might want to increase this parameter in QMC > Scheduler > Engine Timeout (in mins)