Do not allow persistence callbacks to throw Exception

Do not allow persistence callbacks to throw Exception

Since we are indirecting through an executor, we are forced
to wrap any exception -- just do not bother, as the callbacks
are executed in the context of an actor anyway (dealing with
RuntimeExceptions) and users are not throwing checked exceptions.

Change-Id: I6cea19ab7192fa42ad3c346d554411cd0d558a64
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>