xmlrpc_test: Rename exception instance before working with it
Python 3 unsets the exception variable at the end of an "except"
block to prevent reference cycles and speed up garbage collection.
Store the exception under a different name in order to use it later.
Part of the work for https://fedorahosted.org/freeipa/ticket/4985
Reviewed-By: Martin Basti <firstname.lastname@example.org>
Reviewed-By: Petr Spacek <email@example.com>