SPARK-2893: Do not swallow Exceptions when running a custom kryo registrator

The previous behaviour of swallowing ClassNotFound exceptions when running a custom Kryo registrator could lead to difficult to debug problems later on at serialisation / deserialisation time, see SPARK-2878.  Instead it is better to fail fast.

Added test case.

Author: Graham Dennis <graham.dennis@gmail.com>

Closes #1827 from GrahamDennis/feature/spark-2893 and squashes the following commits:

fbe4cb6 [Graham Dennis] [SPARK-2878]: Update the test case to match the updated exception message
65e53c5 [Graham Dennis] [SPARK-2893]: Improve message when a spark.kryo.registrator fails.
f480d85 [Graham Dennis] [SPARK-2893] Fix typo.
b59d2c2 [Graham Dennis] SPARK-2893: Do not swallow Exceptions when running a custom spark.kryo.registrator

(cherry picked from commit 6b8de0e36c7548046c3b8a57f2c8e7e788dde8cc)
Signed-off-by: Reynold Xin <rxin@apache.org>