radiusproxy plugin: Use str(error) rather than error.message
In Python 3, the "message" attribute has been removed in favor of
calling str() on the error.
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>