Fixed SrpConverter to return null for unknown exceptions. Issue: DATARED...

The PersistenceExceptionTranslator javadoc states
"Do not translate exceptions that are not understand by this translator:	for example, if coming from another persistence framework, or resulting from user code and unrelated to persistence."
However the SrpConverters class will wrap unknown exceptions in a RedisSystemException

This is a similar issue that was fixed for the JEDIS connector in DATAREDIS-82