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 <mbasti@redhat.com>
    Reviewed-By: Petr Spacek <pspacek@redhat.com>