[SPARK-8625] [CORE] Propagate user exceptions in tasks back to driver

This allows clients to retrieve the original exception from the
cause field of the SparkException that is thrown by the driver.
If the original exception is not in fact Serializable then it will
not be returned, but the message and stacktrace will be. (All Java
Throwables implement the Serializable interface, but this is no
guarantee that a particular implementation can actually be
serialized.)

Author: Tom White <tom@cloudera.com>

Closes #7014 from tomwhite/propagate-user-exceptions.