SPARK-1244: Throw exception if map output status exceeds frame size

This is a very small change on top of @andrewor14's patch in #147.

Author: Patrick Wendell <pwendell@gmail.com>
Author: Andrew Or <andrewor14@gmail.com>

Closes #152 from pwendell/akka-frame and squashes the following commits:

e5fb3ff [Patrick Wendell] Reversing test order
393af4c [Patrick Wendell] Small improvement suggested by Andrew Or
8045103 [Patrick Wendell] Breaking out into two tests
2b4e085 [Patrick Wendell] Consolidate Executor use of akka frame size
c9b6109 [Andrew Or] Simplify test + make access to akka frame size more modular
281d7c9 [Andrew Or] Throw exception on spark.akka.frameSize exceeded + Unit tests