[SPARK-1816] LiveListenerBus dies if a listener throws an exception

The solution is to wrap a try / catch / log around the posting of each event to each listener.

Author: Andrew Or <andrewor14@gmail.com>

Closes #759 from andrewor14/listener-die and squashes the following commits:

aee5107 [Andrew Or] Merge branch 'master' of github.com:apache/spark into listener-die
370939f [Andrew Or] Remove two layers of indirection
422d278 [Andrew Or] Explicitly throw an exception instead of 1 / 0
0df0e2a [Andrew Or] Try/catch and log exceptions when posting events
(cherry picked from commit 5c0dafc2c8734a421206a808b73be67b66264dd7)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>