Add getter for unpackErrorHandler in ReceivePack

Add getter for unpackErrorHandler in ReceivePack

The current mechanism for updating the unpack error handler requires
that the error handler is replaced entirely, including communicating
the error to the user. Adding a getter means that delegating
implementations can be constructed so that the error can be processed
before sending to the user, for example for logging.

Change-Id: I4b6f78a041d0f6f5b4076a9a5781565ca3857817
Signed-off-by: Jack Wickham <jwickham@palantir.com>