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.
This loses the stacktrace on exception e -- can we include that in the second parameter of the SparkException as the cause?