Merge pull request #219 from sundeepn/schedulerexception

Scheduler quits when newStage fails

The current scheduler thread does not handle exceptions from newStage stage while launching new jobs. The thread fails on any exception that gets triggered at that level, leaving the cluster hanging with no schduler.

(cherry picked from commit 740922f25d5f81617fbe02c7bcd1610d6426bbef)
Signed-off-by: Reynold Xin <rxin@apache.org>