Merge pull request #450 from kayousterhout/fetch_failures. Closes #450.

Only run ResubmitFailedStages event after a fetch fails

Previously, the ResubmitFailedStages event was called every
200 milliseconds, leading to a lot of unnecessary event processing
and clogged DAGScheduler logs.

Author: Kay Ousterhout <kayousterhout@gmail.com>

== Merge branch commits ==

commit e603784b3a562980e6f1863845097effe2129d3b
Author: Kay Ousterhout <kayousterhout@gmail.com>
Date:   Wed Feb 5 11:34:41 2014 -0800

    Re-add check for empty set of failed stages

commit d258f0ef50caff4bbb19fb95a6b82186db1935bf
Author: Kay Ousterhout <kayousterhout@gmail.com>
Date:   Wed Jan 15 23:35:41 2014 -0800

    Only run ResubmitFailedStages event after a fetch fails

    Previously, the ResubmitFailedStages event was called every
    200 milliseconds, leading to a lot of unnecessary event processing
    and clogged DAGScheduler logs.