[SPARK-12759][Core][Spark should fail fast if --executor-memory is too small for spark to start]

Added an exception to be thrown in UnifiedMemoryManager.scala if the configuration given for executor memory is too low. Also modified the exception message thrown when driver memory is too low.

This patch was tested manually by passing in config options to Spark shell. I also added a test in UnifiedMemoryManagerSuite.scala

Author: Daniel Jalova <djalova@us.ibm.com>

Closes #11255 from djalova/SPARK-12759.